diff --git a/.gitignore b/.gitignore
index f0d86dc6..43287fee 100644
--- a/.gitignore
+++ b/.gitignore
@@ -1,4 +1,5 @@
 /target
+.agent/
 
 # Test files
 test*.ab
@@ -16,4 +17,3 @@ flamegraph.svg
 # Nixos
 .direnv
 /result
-
diff --git a/grammar.ebnf b/grammar.ebnf
index 10502584..8dae6487 100644
--- a/grammar.ebnf
+++ b/grammar.ebnf
@@ -94,7 +94,8 @@ KEYWORD_WHILE = 'while' ;
 ANY_CHAR = ? any character ? ;
 LETTER = 'A'..'Z' | 'a'..'z' ;
 DIGIT = '0'..'9' ;
-TYPE = 'Text' | 'Num' | 'Bool' | 'Null';
+TYPE = SIMPLE_TYPE, { '|', SIMPLE_TYPE } ;
+SIMPLE_TYPE = 'Text' | 'Num' | 'Bool' | 'Null' | 'Int' | '[', TYPE, ']' ;
 UNARY_OP = '-' | KEYWORD_NOT ;
 BINARY_OP = '+' | '-' | '*' | '/' | '%' | KEYWORD_AND | KEYWORD_OR | '==' | '!=' | '<' | '<=' | '>' | '>=' ;
 VISIBILITY = KEYWORD_PUB ;
diff --git a/src/modules/types.rs b/src/modules/types.rs
index 58226009..68b974c4 100644
--- a/src/modules/types.rs
+++ b/src/modules/types.rs
@@ -12,6 +12,7 @@ pub enum Type {
     Num,
     Int,
     Array(Box<Type>),
+    Union(Vec<Type>),
     Generic
 }
 
@@ -23,13 +24,16 @@ impl Type {
 
     pub fn is_subset_of(&self, other: &Type) -> bool {
         match (self, other) {
+            (Type::Generic, Type::Generic) => false,
             (_, Type::Generic) => true,
             (Type::Int, Type::Num) => true,
             (Type::Array(current), Type::Array(other)) => match (&**current, &**other) {
                 (current, Type::Generic) if *current != Type::Generic => true,
                 (Type::Int, Type::Num) => true,
-                _ => false
+                (a, b) => a.is_subset_of(b),
             },
+            (Type::Union(types), other) => types.iter().all(|t| t.is_allowed_in(other)),
+            (other, Type::Union(types)) => types.iter().any(|t| other.is_allowed_in(t)),
             _ => false
         }
     }
@@ -42,6 +46,8 @@ impl Type {
         matches!(self, Type::Array(_))
     }
 
+
+
     pub fn pretty_join(types: &[Self], op: &str) -> String {
         let mut all_types = types.iter().map(|kind| kind.to_string()).collect_vec();
         let last_item = all_types.pop();
@@ -71,6 +77,7 @@ impl Display for Type {
                 } else {
                     write!(f, "[{t}]")
                 },
+            Type::Union(types) => write!(f, "{}", types.iter().map(|t| t.to_string()).join(" | ")),
             Type::Generic => write!(f, "Generic")
         }
     }
@@ -89,6 +96,32 @@ pub fn parse_type(meta: &mut ParserMetadata) -> Result<Type, Failure> {
 
 // Tries to parse the type - if it fails, it fails quietly
 pub fn try_parse_type(meta: &mut ParserMetadata) -> Result<Type, Failure> {
+    let mut left = try_parse_simple_type(meta)?;
+    // Parse union type
+    while token(meta, "|").is_ok() {
+        let right = try_parse_simple_type(meta)?;
+        left = match (left, right) {
+            (Type::Union(mut left_types), Type::Union(mut right_types)) => {
+                left_types.append(&mut right_types);
+                Type::Union(left_types)
+            },
+            (Type::Union(mut left_types), right) => {
+                left_types.push(right);
+                Type::Union(left_types)
+            },
+            (left, Type::Union(mut right_types)) => {
+                let mut left_types = vec![left];
+                left_types.append(&mut right_types);
+                Type::Union(left_types)
+            },
+            (left, right) => Type::Union(vec![left, right])
+        }
+    }
+
+    Ok(left)
+}
+
+fn try_parse_simple_type(meta: &mut ParserMetadata) -> Result<Type, Failure> {
     let tok = meta.get_current_token();
     let res = match tok.clone() {
         Some(matched_token) => {
diff --git a/src/tests/erroring/union_types_mismatch.ab b/src/tests/erroring/union_types_mismatch.ab
new file mode 100644
index 00000000..dca6c41d
--- /dev/null
+++ b/src/tests/erroring/union_types_mismatch.ab
@@ -0,0 +1,8 @@
+// Output
+// 1st argument 'x' of function 'foo' expects type 'Int | Bool', but 'Text' was given
+
+fun foo(x: Int | Bool) {
+    echo x
+}
+
+foo("hello")
diff --git a/src/tests/validity/union_types_valid.ab b/src/tests/validity/union_types_valid.ab
new file mode 100644
index 00000000..e5430980
--- /dev/null
+++ b/src/tests/validity/union_types_valid.ab
@@ -0,0 +1,30 @@
+// Output
+// 1
+// 1
+// hello
+// 1 2
+// 1 0
+// 1 2
+// 1 0
+
+fun foo(x: Int | Bool | Text) {
+    echo x
+}
+
+foo(1)
+foo(true)
+foo("hello")
+
+fun bar(x: [Int] | [Bool]) {
+    echo x
+}
+
+bar([1, 2])
+bar([true, false])
+
+fun baz(x: [Int | Bool]) {
+    echo x
+}
+
+baz([1, 2])
+baz([true, false])