In the past decade, scholarly attention to the theory of affect has been exponentially increasing, as it is applied to a growing number of themes and disciplines. Many analysts have made reference to Deleuze in developing their theories, but the engagement with Deleuze in affect theory has yet to be fully and systematically explored. This thesis offers a vigorous examination of the development of the concept of affect in Deleuze and Guattaris philosophical works. I argue that there is a clear difference between affect as Deleuze articulates it in his readings of Spinoza and affect as he develops it with Guattari in the two volumes of Capitalism and Schizophrenia, and hence that there is no single theory of affect in Deleuze. To show the relation between these two notions of affect, I will adopt a classical approachthe genealogy of conceptsto the study of this new field of affect theory.