The book deals with the complex notion of process worked out by Alfred N. Whitehead, a notion that includes his deep revision of the concepts of time and space. Throughout his whole career, Whitehead emphasized the importance of process for the account of reality. The book discusses Whitehead's concept of process starting from his works in mathematics, logic and epistemology of the natural sciences, in order to pave the way for a better understanding of Whitehead's speculative philosophy. The focus is on the relationship between concrescence and transition, seen as deeply correlated and reciprocally determined. In this perspective, the book offers a reinterpretation of Whitehead's most enigmatic proposal: the event of subjectivity interpreted as a process of subjectivation. This poses the problem of the endurance of subjectivity. The book suggests that this problem is solved by Whitehead through is appropriation of Plato's notion of chora, seen as the "place" of a processual subject.