Concurrent and distributed computing systems play an ever-increasing role in the fast-growing field of application of computer-based technologies. At the same time, the complexity of such systems grows rapidly, making it very hard to guarantee the correctness of their ongoing operations, robustness, or resilience to security threats. A key issue is, therefore, to understand the behavioural characteristics of concurrent systems and, in particular, to provide appropriate mathematical abstractions for capturing their intricate behaviours.