This chapter begins by placing some of the central claims of the "Extended Mind Hypothesis", as introduced by Andy Clark and David Chalmers, in the context of aetiological theories of function and some positions within evolutionary biology. The insights from this discussion are then used to identify a number of possible interpretations of what extensions of the mind are, and what explanatory roles they might play in cognitive inquiries. The chapter also describes the notion of "active" versus "passive" externalism and the notion of features of the environment serving cognitive functions. Further, it explores the constitutiveness theme with the aim of providing some common ground for "parity" and "complementarity" views of extendedness. The chapter concludes by discussing a few paradigms of cognitive extension in the light of the aetiological variety of functionalist argument developed before.