In the world we live in concurrency is the norm. For example, the human body is a massively concurrent system, comprising a huge number of cells, all sim- taneously evolving and independently engaging in their individual biological processing. In addition, in the biological world, truly sequential systems rarely arise. However, they are more common when manmade artefacts are cons- ered. In particular, computer systems are often developed from a sequential perspective. Why is this? The simple reason is that it is easier for us to think about sequential, rather than concurrent, systems. Thus, we use sequentiality as a device to simplify the design process. However, the need for increasingly powerful,—exible and usable computer systems mitigates against simplifying sequentiality assumptions. A good - ample of this is the all-powerful position held by the Internet, which is highly concurrent at many different levels of decomposition. Thus, the modern c- puter scientist (and indeed the modern scientist in general) is forced to think about concurrent systems and the subtle and intricate behaviour that emerges from the interaction of simultaneously evolving components. Over a period of 25 years, or so, the field of concurrency theory has been involved in the development of a set of mathematical techniques that can help system developers to think about and build concurrent systems. These theories are the subject matter of this book.