Th e Modigliani-Miller theorem is a cornerstone of fi nance, with interesting implications for both large corporations and entrepreneurial fi rms. Th e celebrated result is an irrelevance proposition, providing conditions under which a fi rm's fi nancial decisions do not aff ect its value. In Villamil (2008), I review key contributions to the literature on the Modigliani-Miller theorem in the last fi fty years, using the theorem's assumptions to organize the debate about why irrelevance often fails: (i) taxes, (ii) capital market frictions such as transaction costs, asset trade restrictions, or bankruptcy costs, (iii) asymmetric access to credit markets, 2 and (iv) asymmetric information, especially when fi rm fi nancial policy reveals information.In the above-mentioned review, I make two main points. First, the Modigliani-Miller assumptions are important because they set conditions for arbitrage. When taxes, transaction or bankruptcy costs, imperfect information, or other frictions that limit access to credit do not distort markets, investors can costlessly replicate a fi rm's fi nancial actions and 'undo' fi rm decisions. Second, the theorem puts all agents on an equal footing, which raises the important question -what types of friction cause agents to have diff erent market opportunities, information sets, or commitment problems? Th e greatest contribution of the theorem may be this analytical perspective of 'equal footedness.' Indeed, it has motivated decades of research in a quest for why fi rm capital structure is relevant, the topic addressed by this special issue.Th e articles in this volume focus specifi cally on the implications of the theorem for entrepreneurial fi rms, a very interesting task that I expect to only gain in