diff --git a/src/main/kotlin/io/smnp/SMNP.kt b/src/main/kotlin/io/smnp/SMNP.kt
index 0aec9be..3951565 100644
--- a/src/main/kotlin/io/smnp/SMNP.kt
+++ b/src/main/kotlin/io/smnp/SMNP.kt
@@ -1,11 +1,6 @@
 package io.smnp
 
-import io.smnp.api.enumeration.DataType
 import io.smnp.api.environment.Environment
-import io.smnp.api.matcher.Matcher.Companion.listOf
-import io.smnp.api.matcher.Matcher.Companion.ofType
-import io.smnp.api.model.Value
-import io.smnp.api.signature.Signature
 import io.smnp.dsl.ast.parser.RootParser
 import io.smnp.dsl.token.tokenizer.DefaultTokenizer
 import io.smnp.evaluation.evaluator.DefaultEvaluator
@@ -23,10 +18,4 @@ fun main(args: Array<String>) {
 
     val value = evaluator.evaluate(ast.node.children[0], Environment()).value!!
 
-    val signature = Signature.vararg(ofType(DataType.INT), ofType(
-        DataType.BOOL), listOf(DataType.NOTE))
-
-    println(signature)
-
-    println(signature.parse(value.value!! as List<Value>))
 }
\ No newline at end of file
diff --git a/src/main/kotlin/io/smnp/api/callable/Function.kt b/src/main/kotlin/io/smnp/api/callable/Function.kt
index 28c2347..ea0f074 100644
--- a/src/main/kotlin/io/smnp/api/callable/Function.kt
+++ b/src/main/kotlin/io/smnp/api/callable/Function.kt
@@ -2,6 +2,7 @@ package io.smnp.api.callable
 
 import io.smnp.api.environment.Environment
 import io.smnp.api.model.Value
+import io.smnp.error.FunctionInvocationException
 
 abstract class Function(val name: String) {
     private var definitions: List<FunctionDefinition> = mutableListOf()
@@ -15,8 +16,9 @@ abstract class Function(val name: String) {
     fun call(environment: Environment, vararg arguments: Value): Value {
         val (definition, args) = definitions
             .map { Pair(it, it.signature.parse(arguments.toList())) }
-            .first { (_, args) -> args.signatureMatched }
-            // TODO: Throw exception if signature is not matched
+            .firstOrNull { (_, args) -> args.signatureMatched }
+            ?: throw FunctionInvocationException(this, arguments)
+
         return definition.body(environment, args.arguments)
     }
 
diff --git a/src/main/kotlin/io/smnp/api/callable/Method.kt b/src/main/kotlin/io/smnp/api/callable/Method.kt
index 38ecb3a..d654c49 100644
--- a/src/main/kotlin/io/smnp/api/callable/Method.kt
+++ b/src/main/kotlin/io/smnp/api/callable/Method.kt
@@ -3,6 +3,7 @@ package io.smnp.api.callable
 import io.smnp.api.environment.Environment
 import io.smnp.api.matcher.Matcher
 import io.smnp.api.model.Value
+import io.smnp.error.MethodInvocationException
 
 abstract class Method(val typeMatcher: Matcher, val name: String) {
     private var definitions: List<MethodDefinition> = mutableListOf()
@@ -18,8 +19,9 @@ abstract class Method(val typeMatcher: Matcher, val name: String) {
     fun call(environment: Environment, obj: Value, vararg arguments: Value): Value {
         val (definition, args) = definitions
             .map { Pair(it, it.signature.parse(arguments.toList())) }
-            .first { (_, args) -> args.signatureMatched }
-        // TODO: Throw exception if signature is not matched
+            .firstOrNull { (_, args) -> args.signatureMatched }
+            ?: throw MethodInvocationException(this, obj, arguments)
+
         return definition.body(environment, obj, args.arguments)
     }
 
diff --git a/src/main/kotlin/io/smnp/api/signature/ActualSignatureFormatter.kt b/src/main/kotlin/io/smnp/api/signature/ActualSignatureFormatter.kt
new file mode 100644
index 0000000..12d5ffd
--- /dev/null
+++ b/src/main/kotlin/io/smnp/api/signature/ActualSignatureFormatter.kt
@@ -0,0 +1,43 @@
+package io.smnp.api.signature
+
+import io.smnp.api.enumeration.DataType
+import io.smnp.api.model.Value
+
+object ActualSignatureFormatter {
+    fun format(arguments: Array<out Value>, parentheses: Boolean = true): String {
+        val output = mutableListOf<String>()
+        for(argument in arguments) {
+            output.add(when(argument.type) {
+                DataType.LIST -> listTypes(argument.value as List<Value>)
+                DataType.MAP -> mapTypes(argument.value as Map<Value, Value>)
+                else -> argument.type.name.toLowerCase()
+            })
+        }
+
+        return if(parentheses) "(${output.joinToString()})" else output.joinToString()
+    }
+
+    private fun listTypes(list: List<Value>, output: MutableList<String> = mutableListOf()): String {
+        for (item in list) {
+            output.add(when (item.type) {
+                DataType.LIST -> listTypes(item.value as List<Value>)
+                DataType.MAP -> mapTypes(item.value as Map<Value, Value>)
+                else -> item.type.name.toLowerCase()
+            })
+        }
+
+        return "list<${output.toSet().joinToString()}>"
+    }
+
+    private fun mapTypes(map: Map<Value, Value>, output: MutableMap<Value, String> = mutableMapOf()): String {
+        for ((k, v) in map) {
+            output[k] = when (v.type) {
+                DataType.LIST -> listTypes(v.value as List<Value>)
+                DataType.MAP -> mapTypes(v.value as Map<Value, Value>)
+                else -> v.type.name.toLowerCase()
+            }
+        }
+
+        return "map<${output.keys.toSet().joinToString()}><${output.values.toSet().joinToString()}}>"
+    }
+}
\ No newline at end of file
diff --git a/src/main/kotlin/io/smnp/error/FunctionInvocationException.kt b/src/main/kotlin/io/smnp/error/FunctionInvocationException.kt
new file mode 100644
index 0000000..90612eb
--- /dev/null
+++ b/src/main/kotlin/io/smnp/error/FunctionInvocationException.kt
@@ -0,0 +1,9 @@
+package io.smnp.error
+
+import io.smnp.api.callable.Function
+import io.smnp.api.model.Value
+import io.smnp.api.signature.ActualSignatureFormatter.format
+
+class FunctionInvocationException(private val function: Function, private val passedArguments: Array<out Value>) : Exception() {
+    override fun toString() = "Invalid signature: ${function.name}${format(passedArguments)}\nAllowed signatures:\n${function.signature}"
+}
\ No newline at end of file
diff --git a/src/main/kotlin/io/smnp/error/MethodInvocationException.kt b/src/main/kotlin/io/smnp/error/MethodInvocationException.kt
new file mode 100644
index 0000000..f748899
--- /dev/null
+++ b/src/main/kotlin/io/smnp/error/MethodInvocationException.kt
@@ -0,0 +1,9 @@
+package io.smnp.error
+
+import io.smnp.api.callable.Method
+import io.smnp.api.model.Value
+import io.smnp.api.signature.ActualSignatureFormatter.format
+
+class MethodInvocationException(private val method: Method, private val obj: Value, private val passedArguments: Array<out Value>) : Exception() {
+    override fun toString() = "Invalid signature: $obj.${method.name}${format(passedArguments)}\nAllowed signatures:\n${method.signature}"
+}
\ No newline at end of file