In this overview we give a short survey of the way in which the theory of the firm has been formulated within the `mainstream' of economics, both `past' and `present'. One aim is to give an outline of the historical developments that led to the contemporary theories. As to a breakpoint between the periods, 1970 is a convenient, if not entirely accurate, dividing line. The major difference between the theories of the past and the present is that the focus, in terms of the questions asked in the theory, of the post-1970 literature is markedly different from that of the earlier (neoclassical) mainstream theory. The questions the theory seeks to answer have changed from being about how the firm acts in the market, how it prices its outputs or how it combines its inputs, to questions about the firm's existence, boundaries and internal organisation and the role of the entrepreneur. That is, there has been a movement away from the theory of the firm being seen as developing a component of price theory, namely issues to do with firm behaviour, to the theory being concerned with the firm as a subject in its own right.