Modern stream processing engines not only have to process millions of events per second at sub-second latency but also have to cope with constantly changing workloads. Due to the dynamic nature of stream applications where the number of incoming events can strongly vary with time, systems cannot reliably predetermine the amount of required resources. In order to meet guaranteed SLAs as well as utilizing system resources as efficiently as possible, frameworks like Apache Flink have to adapt their resource consumption dynamically. In this talk, we will take a look under the hood and explain how Flink scales stateful application in and out. Starting with the concept of key groups and partionable state, we will cover ways to detect bottlenecks in streaming jobs and discuss efficient strategies how to scale out operators with minimal down-time.