Book excerpt: Active Objects are a programming paradigm that supports a non-competitive, data-driven concurrency model. This renders active object languages to be well-suited for simulation, data race-free programming, and formal verification. Concepts from active objects made their way into languages such as Rust, ABS, Akka, JavaScript, and Go. This is the first comprehensive state-of-art overview on the subject, the invited contributions are written by experts in the areas of distributed systems, formal methods, and programming languages.