diff --git a/src/builtin/modules/mod.rs b/src/builtin/modules/mod.rs
index 708bbbe..1490adc 100644
--- a/src/builtin/modules/mod.rs
+++ b/src/builtin/modules/mod.rs
@@ -27,6 +27,34 @@ pub(crate) struct Module(pub Scope);
 #[derive(Debug, Default)]
 pub(crate) struct Modules(BTreeMap<Identifier, Module>);
 
+#[derive(Debug, Default)]
+pub(crate) struct ModuleConfig(BTreeMap<Identifier, Value>);
+
+impl ModuleConfig {
+    /// Removes and returns element with name
+    pub fn get(&mut self, name: Identifier) -> Option<Value> {
+        self.0.remove(&name)
+    }
+
+    /// If this structure is not empty at the end of
+    /// an `@use`, we must throw an error
+    pub fn is_empty(&self) -> bool {
+        self.0.is_empty()
+    }
+
+    pub fn insert(&mut self, name: Spanned<Identifier>, value: Spanned<Value>) -> SassResult<()> {
+        if self.0.insert(name.node, value.node).is_some() {
+            Err((
+                "The same variable may only be configured once.",
+                name.span.merge(value.span),
+            )
+                .into())
+        } else {
+            Ok(())
+        }
+    }
+}
+
 impl Modules {
     pub fn insert(&mut self, name: Identifier, module: Module, span: Span) -> SassResult<()> {
         if self.0.contains_key(&name) {
diff --git a/src/lib.rs b/src/lib.rs
index 6f20adc..f691da6 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -95,7 +95,7 @@ use peekmore::PeekMore;
 pub use crate::error::{SassError as Error, SassResult as Result};
 pub(crate) use crate::token::Token;
 use crate::{
-    builtin::modules::Modules,
+    builtin::modules::{ModuleConfig, Modules},
     lexer::Lexer,
     output::Css,
     parse::{
@@ -295,6 +295,7 @@ pub fn from_path(p: &str, options: &Options) -> Result<String> {
         content_scopes: &mut Scopes::new(),
         options,
         modules: &mut Modules::default(),
+        module_config: &mut ModuleConfig::default(),
     }
     .parse()
     .map_err(|e| raw_to_parse_error(&map, *e, options.unicode_error_messages))?;
@@ -340,6 +341,7 @@ pub fn from_string(p: String, options: &Options) -> Result<String> {
         content_scopes: &mut Scopes::new(),
         options,
         modules: &mut Modules::default(),
+        module_config: &mut ModuleConfig::default(),
     }
     .parse()
     .map_err(|e| raw_to_parse_error(&map, *e, options.unicode_error_messages))?;
@@ -376,6 +378,7 @@ pub fn from_string(p: String) -> std::result::Result<String, JsValue> {
         content_scopes: &mut Scopes::new(),
         options: &Options::default(),
         modules: &mut Modules::default(),
+        module_config: &mut ModuleConfig::default(),
     }
     .parse()
     .map_err(|e| raw_to_parse_error(&map, *e, true).to_string())?;
diff --git a/src/parse/control_flow.rs b/src/parse/control_flow.rs
index babbd43..cb0816e 100644
--- a/src/parse/control_flow.rs
+++ b/src/parse/control_flow.rs
@@ -54,6 +54,7 @@ impl<'a> Parser<'a> {
                 content_scopes: self.content_scopes,
                 options: self.options,
                 modules: self.modules,
+                module_config: self.module_config,
             }
             .parse_stmt()?;
         } else {
@@ -114,6 +115,7 @@ impl<'a> Parser<'a> {
                                 content_scopes: self.content_scopes,
                                 options: self.options,
                                 modules: self.modules,
+                                module_config: self.module_config,
                             }
                             .parse_stmt()?;
                         } else {
@@ -143,6 +145,7 @@ impl<'a> Parser<'a> {
                                 content_scopes: self.content_scopes,
                                 options: self.options,
                                 modules: self.modules,
+                                module_config: self.module_config,
                             }
                             .parse_stmt();
                         }
@@ -323,6 +326,7 @@ impl<'a> Parser<'a> {
                     content_scopes: self.content_scopes,
                     options: self.options,
                     modules: self.modules,
+                    module_config: self.module_config,
                 }
                 .parse_stmt()?;
                 if !these_stmts.is_empty() {
@@ -346,6 +350,7 @@ impl<'a> Parser<'a> {
                         content_scopes: self.content_scopes,
                         options: self.options,
                         modules: self.modules,
+                        module_config: self.module_config,
                     }
                     .parse_stmt()?,
                 );
@@ -397,6 +402,7 @@ impl<'a> Parser<'a> {
                     content_scopes: self.content_scopes,
                     options: self.options,
                     modules: self.modules,
+                    module_config: self.module_config,
                 }
                 .parse_stmt()?;
                 if !these_stmts.is_empty() {
@@ -420,6 +426,7 @@ impl<'a> Parser<'a> {
                         content_scopes: self.content_scopes,
                         options: self.options,
                         modules: self.modules,
+                        module_config: self.module_config,
                     }
                     .parse_stmt()?,
                 );
@@ -512,6 +519,7 @@ impl<'a> Parser<'a> {
                     content_scopes: self.content_scopes,
                     options: self.options,
                     modules: self.modules,
+                    module_config: self.module_config,
                 }
                 .parse_stmt()?;
                 if !these_stmts.is_empty() {
@@ -535,6 +543,7 @@ impl<'a> Parser<'a> {
                         content_scopes: self.content_scopes,
                         options: self.options,
                         modules: self.modules,
+                        module_config: self.module_config,
                     }
                     .parse_stmt()?,
                 );
diff --git a/src/parse/function.rs b/src/parse/function.rs
index a5a8b19..1f1830a 100644
--- a/src/parse/function.rs
+++ b/src/parse/function.rs
@@ -121,6 +121,7 @@ impl<'a> Parser<'a> {
             content_scopes: self.content_scopes,
             options: self.options,
             modules: self.modules,
+            module_config: self.module_config,
         }
         .parse_stmt()?;
 
diff --git a/src/parse/import.rs b/src/parse/import.rs
index ac252af..cd17e8c 100644
--- a/src/parse/import.rs
+++ b/src/parse/import.rs
@@ -108,6 +108,7 @@ impl<'a> Parser<'a> {
                 content_scopes: self.content_scopes,
                 options: self.options,
                 modules: self.modules,
+                module_config: self.module_config,
             }
             .parse();
         }
diff --git a/src/parse/keyframes.rs b/src/parse/keyframes.rs
index adf792c..4a5caf9 100644
--- a/src/parse/keyframes.rs
+++ b/src/parse/keyframes.rs
@@ -174,6 +174,7 @@ impl<'a> Parser<'a> {
                         content_scopes: self.content_scopes,
                         options: self.options,
                         modules: self.modules,
+                        module_config: self.module_config,
                     })
                     .parse_keyframes_selector()?;
 
@@ -210,6 +211,7 @@ impl<'a> Parser<'a> {
             content_scopes: self.content_scopes,
             options: self.options,
             modules: self.modules,
+            module_config: self.module_config,
         }
         .parse_stmt()?;
 
diff --git a/src/parse/media.rs b/src/parse/media.rs
index 2a0c036..61f124a 100644
--- a/src/parse/media.rs
+++ b/src/parse/media.rs
@@ -25,16 +25,6 @@ impl<'a> Parser<'a> {
         Ok(false)
     }
 
-    pub fn expect_char(&mut self, c: char) -> SassResult<()> {
-        if let Some(Token { kind, .. }) = self.toks.peek() {
-            if *kind == c {
-                self.toks.next();
-                return Ok(());
-            }
-        }
-        Err((format!("expected \"{}\".", c), self.span_before).into())
-    }
-
     pub fn scan_char(&mut self, c: char) -> bool {
         if let Some(Token { kind, .. }) = self.toks.peek() {
             if *kind == c {
diff --git a/src/parse/mixin.rs b/src/parse/mixin.rs
index e0c0d63..6fcc1d6 100644
--- a/src/parse/mixin.rs
+++ b/src/parse/mixin.rs
@@ -183,6 +183,7 @@ impl<'a> Parser<'a> {
             content_scopes: self.content_scopes,
             options: self.options,
             modules: self.modules,
+            module_config: self.module_config,
         }
         .parse_stmt()?;
 
@@ -245,6 +246,7 @@ impl<'a> Parser<'a> {
                     content_scopes: self.scopes,
                     options: self.options,
                     modules: self.modules,
+                    module_config: self.module_config,
                 }
                 .parse_stmt()?
             } else {
diff --git a/src/parse/mod.rs b/src/parse/mod.rs
index 45b67ca..7a29a34 100644
--- a/src/parse/mod.rs
+++ b/src/parse/mod.rs
@@ -12,7 +12,8 @@ use crate::{
     },
     builtin::modules::{
         declare_module_color, declare_module_list, declare_module_map, declare_module_math,
-        declare_module_meta, declare_module_selector, declare_module_string, Module, Modules,
+        declare_module_meta, declare_module_selector, declare_module_string, Module, ModuleConfig,
+        Modules,
     },
     error::SassResult,
     lexer::Lexer,
@@ -94,6 +95,7 @@ pub(crate) struct Parser<'a> {
     pub options: &'a Options<'a>,
 
     pub modules: &'a mut Modules,
+    pub module_config: &'a mut ModuleConfig,
 }
 
 impl<'a> Parser<'a> {
@@ -113,6 +115,99 @@ impl<'a> Parser<'a> {
         Ok(stmts)
     }
 
+    pub fn expect_char(&mut self, c: char) -> SassResult<()> {
+        match self.toks.peek() {
+            Some(Token { kind, pos }) if *kind == c => {
+                self.span_before = *pos;
+                self.toks.next();
+                Ok(())
+            }
+            Some(Token { pos, .. }) => Err((format!("expected \"{}\".", c), *pos).into()),
+            None => Err((format!("expected \"{}\".", c), self.span_before).into()),
+        }
+    }
+
+    pub fn consume_char_if_exists(&mut self, c: char) {
+        if let Some(Token { kind, .. }) = self.toks.peek() {
+            if *kind == c {
+                self.toks.next();
+            }
+        }
+    }
+
+    fn parse_module_alias(&mut self) -> SassResult<Option<String>> {
+        if let Some(Token { kind: 'a', .. }) | Some(Token { kind: 'A', .. }) = self.toks.peek() {
+            let mut ident = peek_ident_no_interpolation(self.toks, false, self.span_before)?;
+            ident.node.make_ascii_lowercase();
+            if ident.node != "as" {
+                return Err(("expected \";\".", ident.span).into());
+            }
+
+            self.whitespace_or_comment();
+
+            if let Some(Token { kind: '*', .. }) = self.toks.peek() {
+                self.toks.next();
+                return Ok(Some('*'.to_string()));
+            } else {
+                let name = self.parse_identifier_no_interpolation(false)?;
+
+                return Ok(Some(name.node));
+            }
+        }
+
+        Ok(None)
+    }
+
+    fn parse_module_config(&mut self) -> SassResult<ModuleConfig> {
+        let mut config = ModuleConfig::default();
+
+        if let Some(Token { kind: 'w', .. }) | Some(Token { kind: 'W', .. }) = self.toks.peek() {
+            let mut ident = peek_ident_no_interpolation(self.toks, false, self.span_before)?;
+            ident.node.make_ascii_lowercase();
+            if ident.node != "with" {
+                return Err(("expected \";\".", ident.span).into());
+            }
+
+            self.whitespace_or_comment();
+
+            self.span_before = ident.span;
+
+            self.expect_char('(')?;
+
+            loop {
+                self.whitespace_or_comment();
+                self.expect_char('$')?;
+
+                let name = self.parse_identifier_no_interpolation(false)?;
+
+                self.whitespace_or_comment();
+                self.expect_char(':')?;
+                self.whitespace_or_comment();
+
+                let value = self.parse_value(false, &|toks| match toks.peek() {
+                    Some(Token { kind: ',', .. }) | Some(Token { kind: ')', .. }) => true,
+                    _ => false,
+                })?;
+
+                config.insert(name.map_node(|n| n.into()), value)?;
+
+                match self.toks.next() {
+                    Some(Token { kind: ',', .. }) => {
+                        continue;
+                    }
+                    Some(Token { kind: ')', .. }) => {
+                        break;
+                    }
+                    Some(..) | None => {
+                        return Err(("expected \")\".", self.span_before).into());
+                    }
+                }
+            }
+        }
+
+        Ok(config)
+    }
+
     /// Returns any multiline comments that may have been found
     /// while loading modules
     #[allow(clippy::eval_order_dependence)]
@@ -156,44 +251,14 @@ impl<'a> Parser<'a> {
 
                     self.whitespace_or_comment();
 
-                    let mut module_alias: Option<String> = None;
+                    let module_alias = self.parse_module_alias()?;
 
-                    match self.toks.peek() {
-                        Some(Token { kind: ';', .. }) => {
-                            self.toks.next();
-                        }
-                        Some(Token { kind: 'a', .. }) | Some(Token { kind: 'A', .. }) => {
-                            let mut ident =
-                                peek_ident_no_interpolation(self.toks, false, self.span_before)?;
-                            ident.node.make_ascii_lowercase();
-                            if ident.node != "as" {
-                                return Err(("expected \";\".", ident.span).into());
-                            }
+                    self.whitespace_or_comment();
 
-                            self.whitespace_or_comment();
+                    let mut config = self.parse_module_config()?;
 
-                            let name_span;
-
-                            if let Some(Token { kind: '*', pos }) = self.toks.peek() {
-                                name_span = *pos;
-                                self.toks.next();
-                                module_alias = Some('*'.to_string());
-                            } else {
-                                let name = self.parse_identifier_no_interpolation(false)?;
-
-                                module_alias = Some(name.node);
-                                name_span = name.span;
-                            }
-
-                            if !matches!(self.toks.next(), Some(Token { kind: ';', .. })) {
-                                return Err(("expected \";\".", name_span).into());
-                            }
-                        }
-                        Some(Token { kind: 'w', .. }) | Some(Token { kind: 'W', .. }) => {
-                            todo!("with")
-                        }
-                        Some(..) | None => return Err(("expected \";\".", span).into()),
-                    }
+                    self.whitespace_or_comment();
+                    self.expect_char(';')?;
 
                     let module = match module_name.as_ref() {
                         "sass:color" => declare_module_color(),
@@ -232,19 +297,28 @@ impl<'a> Parser<'a> {
                                         content_scopes: self.content_scopes,
                                         options: self.options,
                                         modules: self.modules,
+                                        module_config: &mut config,
                                     }
                                     .parse()?,
                                 );
 
+                                if !config.is_empty() {
+                                    return Err(("This variable was not declared with !default in the @used module.", span).into());
+                                }
+
                                 Module::new_from_scope(global_scope)
                             } else {
-                                return Err(
-                                    ("Error: Can't find stylesheet to import.", span).into()
-                                );
+                                return Err(("Can't find stylesheet to import.", span).into());
                             }
                         }
                     };
 
+                    // if the config isn't empty here, that means
+                    // variables were passed to a builtin module
+                    if !config.is_empty() {
+                        return Err(("Built-in modules can't be configured.", span).into());
+                    }
+
                     let module_name = match module_alias.as_deref() {
                         Some("*") => {
                             self.global_scope.merge_module(module);
@@ -599,6 +673,7 @@ impl<'a> Parser<'a> {
                 content_scopes: self.content_scopes,
                 options: self.options,
                 modules: self.modules,
+                module_config: self.module_config,
             },
             allows_parent,
             true,
@@ -657,10 +732,11 @@ impl<'a> Parser<'a> {
 
     pub fn parse_interpolation(&mut self) -> SassResult<Spanned<Value>> {
         let val = self.parse_value(true, &|_| false)?;
-        match self.toks.next() {
-            Some(Token { kind: '}', .. }) => {}
-            Some(..) | None => return Err(("expected \"}\".", val.span).into()),
-        }
+
+        self.span_before = val.span;
+
+        self.expect_char('}')?;
+
         Ok(val.map_node(Value::unquote))
     }
 
@@ -899,6 +975,7 @@ impl<'a> Parser<'a> {
             content_scopes: self.content_scopes,
             options: self.options,
             modules: self.modules,
+            module_config: self.module_config,
         }
         .parse_stmt()?
         .into_iter()
@@ -944,14 +1021,14 @@ impl<'a> Parser<'a> {
             content_scopes: self.content_scopes,
             options: self.options,
             modules: self.modules,
+            module_config: self.module_config,
         }
         .parse_selector(false, true, String::new())?;
 
+        // todo: this might be superfluous
         self.whitespace();
 
-        if let Some(Token { kind: ';', .. }) = self.toks.peek() {
-            self.toks.next();
-        }
+        self.consume_char_if_exists(';');
 
         let extend_rule = ExtendRule::new(value.clone(), is_optional, self.span_before);
 
diff --git a/src/parse/value/parse.rs b/src/parse/value/parse.rs
index 5a826ab..1962fba 100644
--- a/src/parse/value/parse.rs
+++ b/src/parse/value/parse.rs
@@ -199,6 +199,7 @@ impl<'a> Parser<'a> {
             content_scopes: self.content_scopes,
             options: self.options,
             modules: self.modules,
+            module_config: self.module_config,
         }
         .parse_value(in_paren, predicate)
     }
@@ -224,6 +225,7 @@ impl<'a> Parser<'a> {
             content_scopes: self.content_scopes,
             options: self.options,
             modules: self.modules,
+            module_config: self.module_config,
         }
         .parse_value(in_paren, &|_| false)
     }
diff --git a/src/parse/variable.rs b/src/parse/variable.rs
index ad08142..78df9b2 100644
--- a/src/parse/variable.rs
+++ b/src/parse/variable.rs
@@ -39,13 +39,24 @@ impl<'a> Parser<'a> {
         } = self.parse_variable_value()?;
 
         if default {
+            let config_val = self.module_config.get(ident);
             if self.at_root && !self.flags.in_control_flow() {
                 if !self.global_scope.var_exists(ident) {
-                    let value = self.parse_value_from_vec(val_toks, true)?.node;
+                    let value = if let Some(config_val) = config_val {
+                        config_val
+                    } else {
+                        self.parse_value_from_vec(val_toks, true)?.node
+                    };
+
                     self.global_scope.insert_var(ident, value);
                 }
             } else {
-                let value = self.parse_value_from_vec(val_toks, true)?.node;
+                let value = if let Some(config_val) = config_val {
+                    config_val
+                } else {
+                    self.parse_value_from_vec(val_toks, true)?.node
+                };
+
                 if global && !self.global_scope.var_exists(ident) {
                     self.global_scope.insert_var(ident, value.clone());
                 }
diff --git a/src/value/mod.rs b/src/value/mod.rs
index d9b13b3..7e7291a 100644
--- a/src/value/mod.rs
+++ b/src/value/mod.rs
@@ -543,6 +543,7 @@ impl Value {
             content_scopes: parser.content_scopes,
             options: parser.options,
             modules: parser.modules,
+            module_config: parser.module_config,
         }
         .parse_selector(allows_parent, true, String::new())?
         .0)
diff --git a/tests/use.rs b/tests/use.rs
index d30af39..035a86d 100644
--- a/tests/use.rs
+++ b/tests/use.rs
@@ -191,3 +191,65 @@ fn use_idempotent_builtin() {
         input
     );
 }
+
+#[test]
+fn use_with_simple() {
+    let input = "@use \"use_with_simple\" with ($a: red);\na {\n color: use_with_simple.$a;\n}";
+    tempfile!("use_with_simple.scss", "$a: green !default;");
+    assert_eq!(
+        "a {\n  color: red;\n}\n",
+        &grass::from_string(input.to_string(), &grass::Options::default()).expect(input)
+    );
+}
+
+#[test]
+fn use_as_with() {
+    let input = "@use \"use_as_with\" as module with ($a: red);\na {\n color: module.$a;\n}";
+    tempfile!("use_as_with.scss", "$a: green !default;");
+    assert_eq!(
+        "a {\n  color: red;\n}\n",
+        &grass::from_string(input.to_string(), &grass::Options::default()).expect(input)
+    );
+}
+
+#[test]
+fn use_whitespace_and_comments() {
+    let input = "@use  /**/  \"use_whitespace_and_comments\"  /**/  as  /**/  foo  /**/  with  /**/  (  /**/  $a  /**/  :  /**/  red  /**/  )  /**/  ;";
+    tempfile!(
+        "use_whitespace_and_comments.scss",
+        "$a: green !default; a { color: $a }"
+    );
+    assert_eq!(
+        "a {\n  color: red;\n}\n",
+        &grass::from_string(input.to_string(), &grass::Options::default()).expect(input)
+    );
+}
+
+#[test]
+fn use_with_builtin_module() {
+    let input = "@use \"sass:math\" with ($e: 2.7);";
+
+    assert_err!("Error: Built-in modules can't be configured.", input);
+}
+
+#[test]
+fn use_with_variable_never_used() {
+    let input = "@use \"use_with_variable_never_used\" with ($a: red);";
+    tempfile!("use_with_variable_never_used.scss", "");
+
+    assert_err!(
+        "Error: This variable was not declared with !default in the @used module.",
+        input
+    );
+}
+
+#[test]
+fn use_with_same_variable_multiple_times() {
+    let input = "@use \"use_with_same_variable_multiple_times\" as foo with ($a: b, $a: c);";
+    tempfile!("use_with_same_variable_multiple_times.scss", "");
+
+    assert_err!(
+        "Error: The same variable may only be configured once.",
+        input
+    );
+}