This document provides an overview of IoT discovery, which is a service discovery mechanism for announcing the availability of context sources via NGSI-9. It describes the key components and operations of NGSI-9 including registering context sources, discovering available context sources, subscribing for availability notifications, updating subscriptions, and receiving notifications. The target users of IoT discovery are described as well as how it fits into the overall FIWARE IoT architecture as an implementation of the configuration manager generic enabler.