This essay proposes that What Is Philosophy? (1991), written in collaboration with Félix Guattari, not only presents a summary of Gilles Deleuze’s late creative period and, to some extent, a recapitulation of his entire oeuvre but also constitutes his third masterwork. The essay begins by tracing Deleuze’s three periods, especially the development of his thought during the last period, and the process of writing his final book. Then it explores the inextricable connection between the method of creation that results from What Is Philosophy? and its stylistic devices. Through its refined composition and succinct prose, the book puts itself at the service of the approach that leads to bringing forth philosophical concepts, and it attunes the reader to the creative activity. Thus, like The Logic of Sense (1969) and A Thousand Plateaus (1980), it forms a masterwork in which Deleuze’s philosophical efforts that spanned a whole period are condensed and mobilized such that while reading one finds oneself exposed to these efforts and is forced to continue them.