diff --git a/src/core/compileAssign.go b/src/core/compileAssign.go
index 1ce3d1e..24b2be2 100644
--- a/src/core/compileAssign.go
+++ b/src/core/compileAssign.go
@@ -2,13 +2,13 @@ package core
 
 import (
 	"git.urbach.dev/cli/q/src/ast"
-	"git.urbach.dev/cli/q/src/errors"
 	"git.urbach.dev/cli/q/src/token"
 )
 
 // compileAssign compiles an assignment.
 func (f *Function) compileAssign(node *ast.Assign) error {
 	left := node.Expression.Children[0]
+	right := node.Expression.Children[1]
 
 	if left.Token.Kind == token.Array {
 		return f.compileStoreArray(node)
@@ -18,11 +18,9 @@ func (f *Function) compileAssign(node *ast.Assign) error {
 		return f.compileStoreField(node)
 	}
 
-	right := node.Expression.Children[1]
-
 	if left.IsLeaf() {
 		return f.define(left, right, true)
 	}
 
-	return errors.New(&NotImplemented{Subject: "multi-assignment"}, f.File, node.Expression.Source().StartPos)
+	return f.defineMulti(left, right, true)
 }
\ No newline at end of file
diff --git a/src/core/compileDefinition.go b/src/core/compileDefinition.go
index 2ac8a22..42650f7 100644
--- a/src/core/compileDefinition.go
+++ b/src/core/compileDefinition.go
@@ -13,11 +13,5 @@ func (f *Function) compileDefinition(node *ast.Define) error {
 		return f.define(left, right, false)
 	}
 
-	rightValue, err := f.evaluate(right)
-
-	if err != nil {
-		return err
-	}
-
-	return f.compose(left, rightValue)
+	return f.defineMulti(left, right, false)
 }
\ No newline at end of file
diff --git a/src/core/compose.go b/src/core/defineMulti.go
similarity index 68%
rename from src/core/compose.go
rename to src/core/defineMulti.go
index 96e544a..ae90fc7 100644
--- a/src/core/compose.go
+++ b/src/core/defineMulti.go
@@ -7,16 +7,21 @@ import (
 	"git.urbach.dev/cli/q/src/types"
 )
 
-// compose creates SSA values from expressions and composes structs from their individual fields.
-func (f *Function) compose(left *expression.Expression, rightValue ssa.Value) error {
+// defineMulti creates SSA values from expressions and composes structs from their individual fields.
+func (f *Function) defineMulti(left *expression.Expression, right *expression.Expression, isAssign bool) error {
+	rightValue, err := f.evaluate(right)
+
+	if err != nil {
+		return err
+	}
+
+	fn := rightValue.(*ssa.Call).Func
 	leaves := make([]*expression.Expression, 0, 2)
 
 	for leaf := range left.Leaves() {
 		leaves = append(leaves, leaf)
 	}
 
-	fn := rightValue.(*ssa.Call).Func
-
 	if len(leaves) != len(fn.Typ.Output) {
 		return errors.New(&DefinitionCountMismatch{Function: fn.String(), Count: len(leaves), ExpectedCount: len(fn.Typ.Output)}, f.File, left.Source().StartPos)
 	}
@@ -32,12 +37,28 @@ func (f *Function) compose(left *expression.Expression, rightValue ssa.Value) er
 			continue
 		}
 
-		_, exists := f.Block().FindIdentifier(name)
+		leftValue, exists := f.Block().FindIdentifier(name)
 
-		if exists {
+		if !isAssign && exists {
 			return errors.New(&VariableAlreadyExists{Name: name}, f.File, identifier.Source().StartPos)
 		}
 
+		if isAssign {
+			if !exists {
+				return errors.New(&UnknownIdentifier{Name: name}, f.File, identifier.Source().StartPos)
+			}
+
+			phi, isPhi := leftValue.(*ssa.Phi)
+
+			if isPhi && phi.IsPartiallyUndefined() {
+				return errors.New(&PartiallyUnknownIdentifier{Name: name}, f.File, identifier.Source().StartPos)
+			}
+
+			if !types.Is(fn.Typ.Output[i], leftValue.Type()) {
+				return errors.New(&TypeMismatch{Encountered: fn.Typ.Output[i].Name(), Expected: leftValue.Type().Name()}, f.File, right.Source().StartPos)
+			}
+		}
+
 		structure, isStructType := types.Unwrap(fn.Typ.Output[i]).(*types.Struct)
 
 		if !isStructType {
diff --git a/tests/multi-define-assign.q b/tests/multi-define-assign.q
new file mode 100644
index 0000000..38387a5
--- /dev/null
+++ b/tests/multi-define-assign.q
@@ -0,0 +1,23 @@
+main() {
+	a, b, c, d := reverse4(1, 2, 3, 4)
+	assert a == 4
+	assert b == 3
+	assert c == 2
+	assert d == 1
+
+	a, b, c, d = reverse4(10, 20, 30, 40)
+	assert a == 40
+	assert b == 30
+	assert c == 20
+	assert d == 10
+
+	a, _, c, _ = reverse4(100, 200, 300, 400)
+	assert a == 400
+	assert b == 30
+	assert c == 200
+	assert d == 10
+}
+
+reverse4(x int, y int, z int, w int) -> (int, int, int, int) {
+	return w, z, y, x
+}
\ No newline at end of file
diff --git a/tests/tests_test.go b/tests/tests_test.go
index 455ae96..51addb3 100644
--- a/tests/tests_test.go
+++ b/tests/tests_test.go
@@ -81,6 +81,7 @@ var tests = []run{
 	{"return-3", "", "", 0},
 	{"return-4", "", "", 0},
 	{"return-string", "", "Hello\n", 0},
+	{"multi-define-assign", "", "", 0},
 	{"ignore-error", "", "", 0},
 	{"ignore-unused-field", "", "", 0},
 	{"slice", "", "Hello World", 0},