Processes are introduced in this chapter. In the wake of a discussion of the notion of change and the recognition of the relational character of many processes, any idea of introducing occurrents on the basis of a distinction between intrinsic and Cambridge changes in continuants is abandoned. The understanding of occurrents as temporally extended processes—causings rather than relata of a dyadic causal relation—based on the analysis of processes implicit in the articulation of thermodynamics in the second half of the nineteenth century is conceptually more illuminating. The chapter continues with a development the mereological features of processes. Mereological properties include distinguishing temporal parts, parts arising from a consideration of the parts of bodies involved and parts of complex processes such as the elementary reactions underlying complex chemical reactions. The relational character of processes (arising from the several bodies typically involved in processes) and their modal character are taken up in the final sections. A distinction is made between accomplishments and activities, and an analogy with the constitution relation between individuals and their constitutive quantities is suggested by construing continuant events as constituted of processes.