We usually think of signs as fixed relations: a red light signifies ‘Stop’. In his bold new book, James Williams argues that signs are processes: you see the red light and think ‘should I stop?’, triggering a creative response. Williams develops this new process philosophy of signs through a formal model , in contrast to earlier structuralist definitions. He draws on the philosophies of Deleuze and Whitehead, criticises earlier work on the sign in biology by Jakob von Uexkull, and connects to contemporary work on process in the philosophy of biology by John Dupre. The process model has wide applications in the arts, humanities and social sciences, and informs their critical debates with science. In defining the sign as essentially political, this radical definition of the sign opens up new possibilities for social and political critique.