Using the example of an MQTT-based large-scale IoT project which is already in production, Florian Raschbichler shares the lessons learned that have accumulated over more than five years. From the initial architectural phase, via the proper use of broker and client implementations, to debugging best practices after the go-live, typical pitfalls and established solution strategies are highlighted. The talk shows proven ways and means of implementing projects with the highest scaling requirements using the de facto standard protocol for IoT.