---
layout: page
title: User-defined functions 
tagline: Create custom ksqlDB functions 
description: ksqlDB lets you add new functions using Java hooks
keywords: function, user-defined function, aggregation, table  
---

# User-defined functions

ksqlDB comes with a broad range of built-in functions out of the box.
But what happens when you need to use a function that ksqlDB doesn't have?

User-defined functions let you add new functions using Java hooks. Learn
how to use them in the [how-to guide](/how-to-guides/create-a-user-defined-function),
or look up critical information about them in the
[reference documentation](/reference/user-defined-functions/).