These lecture notes concern the basics of the theory of process behaviour. First the concept of a (labelled) transition system receives ample treatment and then the following issues concerning process behaviour are elaborated in the setting of transition systems: (i) concurrency and interaction, (ii) abstraction, (iii) sequential composition, alternative composition, and iteration, (iv) expressions that represent process behaviours, and (v) structural operational semantics and equational laws for such expressions. To quicken an intuitive understanding, direct connections with programs and automata are established wherever appropriate. For the interested reader, direct connections with Petri nets are also established. In each chapter, except the last one, it shows that what has been dealt with so far still has certain limitations. Each time, the next chapter is devoted to reducing the limitations concerned.