diff --git a/main.go b/main.go
index a7d48cf..f9f4505 100644
--- a/main.go
+++ b/main.go
@@ -38,4 +38,15 @@ func main() {
 	}
 
 	log.Printf("Parsed:\n%+#v\n\n", parsed)
+
+	errors := validator(parsed)
+	if len(errors) != 0 {
+		for _, err = range errors {
+			if c, ok := err.(CompilerError); ok {
+				log.Fatalln(err, "\nv- here\n"+string([]rune(string(content))[c.Position:]))
+			}
+
+			log.Fatalln(err)
+		}
+	}
 }
diff --git a/parser.go b/parser.go
index c1cb15a..8f8c46e 100644
--- a/parser.go
+++ b/parser.go
@@ -20,6 +20,10 @@ type Type struct {
 	Value any
 }
 
+const STRING_TYPE_NAME = "string"
+
+var STRING_TYPE = Type{Type: Type_Named, Value: STRING_TYPE_NAME}
+
 type NamedType struct {
 	TypeName string
 }
@@ -46,6 +50,10 @@ type Statement struct {
 	Value any
 }
 
+type ExpressionStatement struct {
+	Expression Expression
+}
+
 type BlockStatement struct {
 	Block Block
 }
@@ -55,8 +63,9 @@ type ReturnStatement struct {
 }
 
 type DeclareLocalVariableStatement struct {
-	Variable    string
-	Initializer *Expression
+	Variable     string
+	VariableType Type
+	Initializer  *Expression
 }
 
 type ExpressionType uint32
@@ -70,8 +79,9 @@ const (
 )
 
 type Expression struct {
-	Type  ExpressionType
-	Value any
+	Type      ExpressionType
+	Value     any
+	ValueType Type
 }
 
 type AssignmentExpression struct {
@@ -107,8 +117,14 @@ type TupleExpression struct {
 	Members []Expression
 }
 
+type Local struct {
+	Name string
+	Type Type
+}
+
 type Block struct {
 	Statements []Statement
+	Locals     map[string]Local
 }
 
 type ParsedParameter struct {
@@ -474,7 +490,7 @@ func (p *Parser) tryDeclareLocalVariableStatement() (*Statement, error) {
 
 	token := pCopy.nextToken()
 	if token.Type == Type_Separator && token.Value.(Separator) == Separator_Semicolon {
-		return &Statement{Type: Statement_DeclareLocalVariable, Value: DeclareLocalVariableStatement{Variable: variableName, Initializer: nil}}, nil
+		return &Statement{Type: Statement_DeclareLocalVariable, Value: DeclareLocalVariableStatement{Variable: variableName, VariableType: *variableType, Initializer: nil}}, nil
 	}
 
 	if token.Type != Type_Operator || token.Value.(Operator) != Operator_Equals {
@@ -494,7 +510,7 @@ func (p *Parser) tryDeclareLocalVariableStatement() (*Statement, error) {
 	}
 
 	*p = pCopy
-	return &Statement{Type: Statement_DeclareLocalVariable, Value: DeclareLocalVariableStatement{Variable: variableName, Initializer: initializer}}, nil
+	return &Statement{Type: Statement_DeclareLocalVariable, Value: DeclareLocalVariableStatement{Variable: variableName, VariableType: *variableType, Initializer: initializer}}, nil
 }
 
 func (p *Parser) expectStatement() (*Statement, error) {
diff --git a/validator.go b/validator.go
new file mode 100644
index 0000000..6bd412d
--- /dev/null
+++ b/validator.go
@@ -0,0 +1,152 @@
+package main
+
+import (
+	"errors"
+)
+
+func createError(message string) error {
+	// TODO: pass token and get actual token position
+	return errors.New(message)
+}
+
+func validateImport(imp *Import) []error {
+	// TODO
+	return nil
+}
+
+func validateExpression(expr *Expression, block *Block) []error {
+	var errors []error
+
+	switch expr.Type {
+	case Expression_Assignment:
+		assignment := expr.Value.(AssignmentExpression)
+		var local Local
+		var ok bool
+		if local, ok = block.Locals[assignment.Variable]; !ok {
+			errors = append(errors, createError("Assignment to undeclared variable "+assignment.Variable))
+			return errors
+		}
+
+		valueErrors := validateExpression(&assignment.Value, block)
+		if len(valueErrors) != 0 {
+			errors = append(errors, valueErrors...)
+			return errors
+		}
+
+		// TODO: check if assignment is valid
+		expr.ValueType = local.Type
+	case Expression_Literal:
+		literal := expr.Value.(LiteralExpression)
+
+		switch literal.Literal.Type {
+		case Literal_Boolean:
+		case Literal_Number:
+			expr.ValueType = Type{Type: Type_Primitive, Value: literal.Literal.Primitive}
+		case Literal_String:
+			expr.ValueType = STRING_TYPE
+		}
+	case Expression_VariableReference:
+		reference := expr.Value.(VariableReferenceExpression)
+		var local Local
+		var ok bool
+		if local, ok = block.Locals[reference.Variable]; !ok {
+			errors = append(errors, createError("Reference to undeclared variable "+reference.Variable))
+			return errors
+		}
+
+		expr.ValueType = local.Type
+	case Expression_Arithmetic:
+		arithmethic := expr.Value.(ArithmeticExpression)
+
+		errors = append(errors, validateExpression(&arithmethic.Left, block)...)
+		errors = append(errors, validateExpression(&arithmethic.Right, block)...)
+
+		if len(errors) != 0 {
+			return errors
+		}
+
+		// TODO: validate types compatible and determine result type
+	case Expression_Tuple:
+		tuple := expr.Value.(TupleExpression)
+
+		var types []Type
+		for _, member := range tuple.Members {
+			memberErrors := validateExpression(&member, block)
+			if len(memberErrors) != 0 {
+				errors = append(errors, memberErrors...)
+				continue
+			}
+
+			types = append(types, member.ValueType)
+		}
+
+		if len(errors) != 0 {
+			return errors
+		}
+
+		expr.ValueType = Type{Type: Type_Tuple, Value: TupleType{Types: types}}
+	}
+
+	return errors
+}
+
+func validateStatement(stmt *Statement, block *Block) []error {
+	var errors []error
+
+	switch stmt.Type {
+	case Statement_Expression:
+		expression := stmt.Value.(ExpressionStatement)
+		errors = append(errors, validateExpression(&expression.Expression, block)...)
+	case Statement_Block:
+		block := stmt.Value.(BlockStatement)
+		errors = append(errors, validateBlock(&block.Block)...)
+	case Statement_Return:
+		ret := stmt.Value.(ReturnStatement)
+		if ret.Value != nil {
+			errors = append(errors, validateExpression(ret.Value, block)...)
+		}
+	case Statement_DeclareLocalVariable:
+		dlv := stmt.Value.(DeclareLocalVariableStatement)
+		if dlv.Initializer != nil {
+			errors = append(errors, validateExpression(dlv.Initializer, block)...)
+		}
+
+		block.Locals[dlv.Variable] = Local{Name: dlv.Variable, Type: dlv.VariableType}
+	}
+
+	return errors
+}
+
+func validateBlock(block *Block) []error {
+	var errors []error
+
+	block.Locals = make(map[string]Local)
+
+	for _, stmt := range block.Statements {
+		errors = append(errors, validateStatement(&stmt, block)...)
+	}
+
+	return errors
+}
+
+func validateFunction(function *ParsedFunction) []error {
+	var errors []error
+
+	errors = append(errors, validateBlock(&function.Body)...)
+
+	return errors
+}
+
+func validator(file *ParsedFile) []error {
+	var errors []error
+
+	for _, imp := range file.Imports {
+		errors = append(errors, validateImport(&imp)...)
+	}
+
+	for _, function := range file.Functions {
+		errors = append(errors, validateFunction(&function)...)
+	}
+
+	return errors
+}