Synapse is the main Matrix homeserver implementation, and is maintained by the Matrix.org Foundation. One of the lesser-known features of Synapse is its module system, which allows any developer to write custom functionalities for it, and to extend its capabilities beyond the scope of the Matrix specification. In this talk, I will cover the current state of Synapse's module system, which has recently been subject to a big overhaul, as well as how to write and use a simple module.