The theory of the firm deserves to play a prominent role in both the undergraduate and graduate industrial organization curriculum, both because of the vast amount of attention that has been paid to this area over the last four decades, as well as its practical relevance for strategy and antitrust. This lecture briefly presents some background on the theory of the firm in general, and the M-form Hypothesis in particular. The M-form Hypothesis is an important theory of firm structure, developed by Chandler (1962) and Williamson (1975). A mathematical model, discussion section and accompanying lecture slides illustrate the concepts necessary to understand the M-form Hypothesis.