diff --git a/src/builtin/mod.rs b/src/builtin/mod.rs
index f9da88e..df49aea 100644
--- a/src/builtin/mod.rs
+++ b/src/builtin/mod.rs
@@ -23,6 +23,7 @@ lazy_static! {
         let mut m = BTreeMap::new();
         color::register(&mut m);
         meta::register(&mut m);
+        string::register(&mut m);
         m
     };
 }
diff --git a/src/builtin/string.rs b/src/builtin/string.rs
index 8b13789..cb4eeb6 100644
--- a/src/builtin/string.rs
+++ b/src/builtin/string.rs
@@ -1 +1,21 @@
+use std::collections::BTreeMap;
 
+use super::Builtin;
+use crate::value::Value;
+
+pub(crate) fn register(f: &mut BTreeMap<String, Builtin>) {
+    decl!(f "to-upper-case", |args, _| {
+        let s: &Value = arg!(args, 0, "string");
+        match s.eval() {
+            Value::Ident(i, q) => Some(Value::Ident(i.to_ascii_uppercase(), q)),
+            _ => todo!("")
+        }
+    });
+    decl!(f "to-lower-case", |args, _| {
+        let s: &Value = arg!(args, 0, "string");
+        match s.eval() {
+            Value::Ident(i, q) => Some(Value::Ident(i.to_ascii_lowercase(), q)),
+            _ => todo!("")
+        }
+    });
+}
diff --git a/tests/strings.rs b/tests/strings.rs
new file mode 100644
index 0000000..5db0bcd
--- /dev/null
+++ b/tests/strings.rs
@@ -0,0 +1,25 @@
+#![cfg(test)]
+
+#[macro_use]
+mod macros;
+
+test!(
+    uppercase_ident,
+    "a {\n  color: to-upper-case(aBc123);\n}\n",
+    "a {\n  color: ABC123;\n}\n"
+);
+test!(
+    lowercase_ident,
+    "a {\n  color: to-lower-case(AbC123);\n}\n",
+    "a {\n  color: abc123;\n}\n"
+);
+test!(
+    uppercase_named_arg,
+    "a {\n  color: to-upper-case($string: aBc123);\n}\n",
+    "a {\n  color: ABC123;\n}\n"
+);
+test!(
+    lowercase_named_arg,
+    "a {\n  color: to-lower-case($string: AbC123);\n}\n",
+    "a {\n  color: abc123;\n}\n"
+);