diff --git a/src/modules/function/invocation_utils.rs b/src/modules/function/invocation_utils.rs
index 769ccce4..ed8dbdc7 100644
--- a/src/modules/function/invocation_utils.rs
+++ b/src/modules/function/invocation_utils.rs
@@ -39,6 +39,17 @@ fn run_function_with_args(meta: &mut ParserMetadata, mut fun: FunctionDecl, args
     // Check if the function argument types match
     if fun.is_args_typed {
         for (index, (arg_name, arg_type, given_type)) in izip!(fun.arg_names.iter(), fun.arg_types.iter(), args.iter()).enumerate() {
+
+            // union type matching works differently in functions
+            if let Type::Union(union) = &arg_type {
+                if ! union.has(given_type) {
+                    let fun_name = &fun.name;
+                    let ordinal = ordinal_number(index);
+                    return error!(meta, tok, format!("{ordinal} argument of function '{fun_name}' does not allow '{given_type}' in '{arg_type}'"))
+                }
+                continue;
+            }
+
             if !given_type.is_allowed_in(arg_type) {
                 let fun_name = &fun.name;
                 let ordinal = ordinal_number(index);
diff --git a/src/modules/types.rs b/src/modules/types.rs
index 825b8a1f..f93464d1 100644
--- a/src/modules/types.rs
+++ b/src/modules/types.rs
@@ -1,14 +1,19 @@
 use std::fmt::Display;
 
 use heraclitus_compiler::prelude::*;
+use itertools::Itertools;
+use union::UnionType;
 use crate::utils::ParserMetadata;
 
-#[derive(Debug, Clone, PartialEq, Eq, Default)]
+mod union;
+
+#[derive(Debug, Clone, PartialEq, Eq, PartialOrd, Ord, Default)]
 pub enum Type {
     #[default] Null,
     Text,
     Bool,
     Num,
+    Union(UnionType),
     Array(Box<Type>),
     Failable(Box<Type>),
     Generic
@@ -54,7 +59,12 @@ impl Display for Type {
                     write!(f, "[{}]", t)
                 },
             Type::Failable(t) => write!(f, "{}?", t),
-            Type::Generic => write!(f, "Generic")
+            Type::Generic => write!(f, "Generic"),
+
+            Type::Union(types) => {
+                let types: &Vec<Type> = types.into();
+                write!(f, "{}", types.iter().map(|x| format!("{x}")).join(" | "))
+            }
         }
     }
 }
@@ -70,10 +80,8 @@ pub fn parse_type(meta: &mut ParserMetadata) -> Result<Type, Failure> {
         .map_err(|_| Failure::Loud(Message::new_err_at_token(meta, tok).message("Expected a data type")))
 }
 
-// Tries to parse the type - if it fails, it fails quietly
-pub fn try_parse_type(meta: &mut ParserMetadata) -> Result<Type, Failure> {
-    let tok = meta.get_current_token();
-    let res = match tok.clone() {
+fn parse_type_tok(meta: &mut ParserMetadata, tok: Option<Token>) -> Result<Type, Failure> {
+    match tok.clone() {
         Some(matched_token) => {
             match matched_token.word.as_ref() {
                 "Text" => {
@@ -134,10 +142,35 @@ pub fn try_parse_type(meta: &mut ParserMetadata) -> Result<Type, Failure> {
         None => {
             Err(Failure::Quiet(PositionInfo::at_eof(meta)))
         }
-    };
+    }
+}
 
+fn parse_one_type(meta: &mut ParserMetadata, tok: Option<Token>) -> Result<Type, Failure> {
+    let res = parse_type_tok(meta, tok)?;
     if token(meta, "?").is_ok() {
-        return res.map(|t| Type::Failable(Box::new(t)))
+        return Ok(Type::Failable(Box::new(res)))
+    }
+    Ok(res)
+}
+
+// Tries to parse the type - if it fails, it fails quietly
+pub fn try_parse_type(meta: &mut ParserMetadata) -> Result<Type, Failure> {
+    let tok = meta.get_current_token();
+    let res = parse_one_type(meta, tok);
+
+    if token(meta, "|").is_ok() {
+        // is union type
+        let mut unioned = vec![ res? ];
+        loop {
+            match parse_one_type(meta, meta.get_current_token()) {
+                Err(err) => return Err(err),
+                Ok(t) => unioned.push(t)
+            };
+            if token(meta, "|").is_err() {
+                break;
+            }
+        }
+        return Ok(Type::Union(unioned.into()))
     }
 
     res
diff --git a/src/modules/types/union.rs b/src/modules/types/union.rs
new file mode 100644
index 00000000..cd2b22e6
--- /dev/null
+++ b/src/modules/types/union.rs
@@ -0,0 +1,39 @@
+use super::Type;
+
+#[derive(Debug, Clone, PartialEq, Eq, PartialOrd, Ord, Default)]
+pub struct UnionType(pub Vec<Type>);
+
+impl UnionType {
+    pub fn has(&self, other: &Type) -> bool {
+        self.0.iter().find(|x| **x == *other).is_some()
+    }
+}
+
+impl Into<Vec<Type>> for UnionType {
+    fn into(self) -> Vec<Type> {
+        self.0
+    }
+}
+
+impl <'a> Into<&'a Vec<Type>> for &'a UnionType {
+    fn into(self) -> &'a Vec<Type> {
+        &self.0
+    }
+}
+
+impl From<Vec<Type>> for UnionType {
+    fn from(value: Vec<Type>) -> Self {
+        let mut value = value;
+        value.sort();
+        if value.len() < 2 {
+            unreachable!("A union type must have at least two elements")
+        }
+        for typ in &value {
+            if let Type::Union(_) = typ {
+                unreachable!("Union types cannot be nested")
+            }
+        }
+        
+        Self(value)
+    }
+}
diff --git a/src/tests/errors.rs b/src/tests/errors.rs
index dadb73d3..404890a9 100644
--- a/src/tests/errors.rs
+++ b/src/tests/errors.rs
@@ -1,5 +1,7 @@
 use super::test_amber;
 
+mod unions;
+
 #[test]
 #[should_panic(expected = "ERROR: Return type does not match function return type")]
 fn function_with_wrong_typed_return() {
diff --git a/src/tests/errors/unions.rs b/src/tests/errors/unions.rs
new file mode 100644
index 00000000..19639ffb
--- /dev/null
+++ b/src/tests/errors/unions.rs
@@ -0,0 +1,32 @@
+use crate::tests::test_amber;
+
+#[test]
+#[should_panic(expected = "ERROR: 1st argument 'param' of function 'abc' expects type 'Text | Null', but 'Num' was given")]
+fn invalid_union_type_eq_normal_type() {
+    let code = r#"
+        fun abc(param: Text | Null) {}
+        abc("")
+        abc(123)
+    "#;
+    test_amber(code, "");
+}
+
+#[test]
+#[should_panic(expected = "ERROR: 1st argument 'param' of function 'abc' expects type 'Text | Null', but 'Num | [Text]' was given")]
+fn invalid_two_unions() {
+    let code = r#"
+        fun abc(param: Text | Null) {}
+        abc(123 as Num | [Text])
+    "#;
+    test_amber(code, "");
+}
+
+#[test]
+#[should_panic(expected = "ERROR: 1st argument 'param' of function 'abc' expects type 'Text | Num | Text? | Num? | [Null]', but 'Null' was given")]
+fn big_union() {
+    let code = r#"
+        fun abc(param: Text | Num | Text? | Num? | [Null]) {}
+        abc(null)
+    "#;
+    test_amber(code, "");
+}
\ No newline at end of file
diff --git a/src/tests/mod.rs b/src/tests/mod.rs
index 1b62c2fb..d8d73155 100644
--- a/src/tests/mod.rs
+++ b/src/tests/mod.rs
@@ -8,6 +8,7 @@ use std::process::{Command, Stdio};
 pub mod cli;
 pub mod errors;
 pub mod extra;
+pub mod types;
 pub mod postprocessor;
 mod stdlib;
 mod validity;
diff --git a/src/tests/types/eq.rs b/src/tests/types/eq.rs
new file mode 100644
index 00000000..a19cb1ea
--- /dev/null
+++ b/src/tests/types/eq.rs
@@ -0,0 +1,34 @@
+use crate::modules::types::Type;
+
+#[test]
+fn normal_types_eq() {
+    let types = vec![Type::Null, Type::Text, Type::Bool, Type::Num, Type::Generic];
+    for typ in types {
+        assert_eq!(typ, typ, "{typ} and {typ} must be equal!");
+    }
+}
+
+#[test]
+fn two_different_normal_types() {
+    assert_ne!(Type::Null, Type::Bool);
+}
+
+#[test]
+fn normal_and_failable_type() {
+    assert_ne!(Type::Failable(Box::new(Type::Text)), Type::Text, "Text? and Text must not be equal!")
+}
+
+#[test]
+fn array_and_normal_type() {
+    assert_ne!(Type::Array(Box::new(Type::Bool)), Type::Bool);
+}
+
+#[test]
+fn array_and_array_of_failables() {
+    assert_ne!(Type::Array(Box::new(Type::Bool)), Type::Array(Box::new(Type::Failable(Box::new(Type::Bool)))));
+}
+
+#[test]
+fn nested_array_normal_array_with_failable() {
+    assert_ne!(Type::Array(Box::new(Type::Array(Box::new(Type::Bool)))), Type::Failable(Box::new(Type::Bool)));
+}
\ No newline at end of file
diff --git a/src/tests/types/mod.rs b/src/tests/types/mod.rs
new file mode 100644
index 00000000..40991751
--- /dev/null
+++ b/src/tests/types/mod.rs
@@ -0,0 +1,2 @@
+pub mod union;
+pub mod eq;
diff --git a/src/tests/types/union.rs b/src/tests/types/union.rs
new file mode 100644
index 00000000..5c961d3f
--- /dev/null
+++ b/src/tests/types/union.rs
@@ -0,0 +1,73 @@
+use crate::modules::types::Type;
+
+#[test]
+fn partially_overlapping_types() {
+    let one = Type::Union(vec![Type::Text, Type::Num]);
+    let two = Type::Union(vec![Type::Num, Type::Null]);
+
+    assert_ne!(one, two, "Text | Num must not be equal to Num | Null!")
+}
+
+#[test]
+fn overlapping_types() {
+    let one = Type::Union(vec![Type::Text, Type::Num]);
+    let two = Type::Union(vec![Type::Text, Type::Num, Type::Null]);
+
+    assert_eq!(one, two, "Text | Num must be equal to Text | Num | Null!")
+}
+
+#[test]
+fn same_union() {
+    let one = Type::Union(vec![Type::Text, Type::Num]);
+    let two = Type::Union(vec![Type::Text, Type::Num]);
+
+    assert_eq!(one, two, "Text | Num must be equal to Text | Num!")
+}
+
+#[test]
+fn empty_union() {
+    let one = Type::Union(vec![]);
+    let two = Type::Union(vec![]);
+
+    assert_eq!(one, two, "If one of unions is empty, it must always be equal to another")
+}
+
+#[test]
+fn empty_and_normal_union() {
+    let one = Type::Union(vec![Type::Text, Type::Num]);
+    let two = Type::Union(vec![]);
+
+    assert_eq!(one, two, "If one of unions is empty, it must always be equal to another")
+}
+
+#[test]
+fn empty_union_and_normal_type() {
+    let one = Type::Union(vec![]);
+    let two = Type::Text;
+
+    assert_ne!(one, two, "An empty union and one type are not equal")
+}
+
+#[test]
+fn big_union() {
+    let one = Type::Union(vec![Type::Text, Type::Text, Type::Text, Type::Text, Type::Text, Type::Text, Type::Text, Type::Num]);
+    let two = Type::Union(vec![Type::Text, Type::Num]);
+
+    assert_eq!(one, two, "Text | Text | ... | Text | Num and Text | Num must be equal!")
+}
+
+#[test]
+fn normal_and_union() {
+    let one = Type::Text;
+    let two = Type::Union(vec![Type::Text, Type::Null]);
+
+    assert_eq!(one, two, "Text and Text | Null must be equal!");
+}
+
+#[test]
+fn normal_not_in_union() {
+    let one = Type::Text;
+    let two = Type::Union(vec![Type::Num, Type::Null]);
+
+    assert_ne!(one, two, "Text and Num | Null must not be equal!");
+}
diff --git a/src/tests/validity/function_with_union_types.ab b/src/tests/validity/function_with_union_types.ab
new file mode 100644
index 00000000..d0797888
--- /dev/null
+++ b/src/tests/validity/function_with_union_types.ab
@@ -0,0 +1,10 @@
+// Output
+// abc
+// 123
+
+fun check(thing: Text | Num): Null {
+    echo thing
+}
+
+check("abc")
+check(123)
\ No newline at end of file
diff --git a/src/tests/validity/union_types.ab b/src/tests/validity/union_types.ab
new file mode 100644
index 00000000..387c0d27
--- /dev/null
+++ b/src/tests/validity/union_types.ab
@@ -0,0 +1,7 @@
+// Output
+// 123
+
+let thingy = "abc" as Text | Num;
+thingy = 123;
+
+echo thingy;
\ No newline at end of file
diff --git a/src/tests/validity/union_types_if.ab b/src/tests/validity/union_types_if.ab
new file mode 100644
index 00000000..5a913a5a
--- /dev/null
+++ b/src/tests/validity/union_types_if.ab
@@ -0,0 +1,19 @@
+// Output
+// is text
+// abc
+// is num
+// 123
+
+fun check(thing: Text | Num): Null {
+    if thing is Text {
+        echo "is text"
+        echo thing
+    }
+    if thing is Num {
+        echo "is num"
+        echo thing
+    }
+}
+
+check("abc")
+check(123)
\ No newline at end of file