Because much of the recent philosophical interest in functions has been motivated by their application in biology and other sciences, most of the ensuing discussions have focused on functional explanations to the neglect of the practical role of functional knowledge. This practical role is essential for understanding how users form plans involving artifacts. We introduce the concept of instrumental function which is intended to capture the features of functional claims that are relevant to practical—in particular, instrumental—reasoning. We discuss the four primary features of an instrumental function ascription, the teleological component of such ascriptions and give a clear definition of malfunction in terms of incapability to perform as well as “normal” tokens of the same type.