This presentation discusses the concept of communication. Communication is seen as unifying, potentially infinite alternating process of the two phases described in the previous chapters, namely publication (write) in Chapter 2 and monitoring (read) in Chapter 3. We start by defining what communication is, what types of communication (e.g. active, re-active) exist, and continue with how to trace communication including actors, messages, channels, time and data, and finally discuss challenges and solutions for multi-channel and multi-agent communication.