The historian Raphael Falk has described the gene as a 'concept in tension' (Falk 2000) - an idea pulled this way and that by the differing demands of different kinds of biological work. Several authors have suggested that in the light of contemporary molecular biology 'gene' is no more than a handy term that acquires a precise meaning only in some specific scientific context in which it is used. Hence the best way to answer the question 'What is a gene?', and the only way to provide a truly philosophical answer to that question is to outline the diversity of conceptions of the gene and the reasons for this diversity. In this essay we draw on the extensive literature in the history of biology to explain how the concept has changed over time in response to the changing demands of the biosciences. In this section we have drawn primarily on the work of Raphael Falk (1986, 1991, 1995, 2000, 2001, 2005, in press), Michael Dietrich (2000a, 2000b), Robert Olby (1974, 1985), Petter Portin (1993), and Michael Morange (1998). When our historical claims are commonplaces that can be found in several of these sources we do not cite specific works in their support. We have also chosen not to explain basic genetic terminology, as this would have occupied much of the chapter. More specialized terms are explained when they cannot be avoided. In the final part of the essay we outline some of the conceptions of the gene current today. The seeds of change are implicit in many of those current conceptions and the future of the gene concept appears set to be at as turbulent as its past.