Biological functions are dispositions or effects a trait has which explain the recent maintenance of the trait under natural selection. This is the approach to functions. The approach is historical because to ascribe a is to make a claim about the past, but the relevant past is the recent past; modem history rather than ancient. The modem history view is not new. It is a point upon which much of the functions literature has been converging for the best part of two decades, and there are implicit or partial statements of the view to be found in many writers. This paper aims to make the position entirely explicit, to show how it emerges from the work of other authors, and to claim that it is the right approach to biological functions. Adopting a modem history position does not solve all the philosophical problems about functions. It deals with a family of questions concerning time and explanation, but there are other difficulties which are quite distinct. The most important of these concern the extent to which functional characterization requires a commitment to some form of adaptationism (Gould and Lewontin 1978). These issues will not be addressed here. Further, as many writers note, function is a highly ambiguous term. It is used in a variety of scientific and philosophical theories, several domains of everyday discourse, and there is probably even a plurality of senses current within biology. This paper is concerned with one core biological sense of the term, which is associated with a particular kind of explanation. In this sense a has some link to an explanation of why the functionally characterized thing exists, in the form it does. Cummins (1975) argued that functions are properly associated with a different explanatory project, that of explaining how a component in a larger system contributes to the system exhibiting some more complex capacity. Following Millikan (1989b) I suggest that both kinds of functions should be recognized,