The concept of emergence—which I define as the (diachronic) construction of functional structures in complex systems that achieve a (synchronic) focus of systematic behaviour as they constrain the behaviour of individual components—plays a crucial role in debates in philosophical reflection on science as a whole (the question of reductionism) as well as in the fields of biology (the status of the organism), social science (the practical subject) and cognitive science (the cognitive subject). In this article I examine how the philosophy of Deleuze and that of Deleuze and Guattari can help us see some of the most important implications of these debates.