In this paper, we describe the conceptual elusiveness of the notion of function as used in engineering practice. We argue that it should be accepted as an ambiguous notion, and then review philosophical argumentations in which engineering functions occur in order to identify the consequences of this ambiguity. Function is a key notion in engineering, yet is used by engineers systematically in a variety of meanings. First, we demonstrate that this ambiguous use is rational for engineers by considering the role of functions in design methods and by analysing the ambiguity in terms of Kuhn's notion of methodological incommensurability. Second, we discuss ontological and mereological analyses of engineering functions and describe a proof that subfunctions cannot formally be taken as parts of the functions they decompose. Engineering functions figure sometimes in philosophical work and are then typically taken as having an unambiguous, well-defined meaning. Finally, we therefore revisit work in philosophy of technology on the dual nature of technical artefacts, in philosophy of science on functional and mechanistic explanations, and in philosophy of biology on biological functions, and explore the consequences of the fact that engineering function is an ambiguous notion. It is argued that one of these consequences may be that also the notion of biological function has an ambiguous meaning.