In mergers and acquisitions opinions are written evaluations of the financial terms of offers for target firms. We use simultaneous equations logit models to identify factors influencing the probabilities of firms obtaining third-party fairness opinions. We find that probabilities are increased by asymmetric information and litigation risk as well as by a variety of uncertainty measures including market values, whether the deal is friendly or the terms are cash, and use of other financial advisory services. We find, in particular, that probabilities have been increased by the Delaware Court's Van Gorkom decision but not by the SEC's Regulation FD.