The object of the present essay is neither to overturn transaction cost economics nor to deny the reality of opportunism. Instead, it is to question the explanatory role of the concept of opportunism in the transaction cost analysis of Oliver Williamson and his followers. Williamson has suggested that potential or actual opportunism emerges as the source of the ‘transaction costs’ involved in monitoring and enforcing contracts. On the contrary, it is shown here that there are several additional and likely sources of contract default or incomplete performance. Williamson’s explanatory concentration on opportunism thus misidentifies the reasons for different hierarchical governance structures in the real world. As these additional sources are ignored, the emphasis on opportunism can impair a true understanding of the inner workings of the firm, and be a misleading guide for practical questions of organizational design and corporate strategy.