This article explores the role of customers in informing competition authorities and courts about the likely effects of proposed mergers. It discusses when, and about what, customers are most likely to be valuable sources of information. It also discusses the potential limitations of customer testimony. Customer views can certainly be informative. However, they are best employed as a complement to, rather than a substitute for, economic analysis that employs more objective evidence. Although the welfare of consumers appears increasingly to have been accepted by competition authorities as the appropriate goal for merger policy, customers will not necessarily have available the information required to predict the economic consequences of a proposed merger. And even when they do, customers may be reluctant reliably and accurately to provide that information, or to express their true opinions, to investigators and before courts. Worse yet, customers may themselves stand to benefit from mergers that are anticompetitive, or may be harmed by mergers that are welfare-enhancing. Appreciating both the strengths and the limitations of customer input is critical to the cause of sound merger enforcement, not only in the U.S., but overseas as well. A growing number of countries, many of whom have less experience or expertise than do competition authorities in the U.S., have begun to adopt merger control policies themselves, and are in the process of developing and implementing investigative best practices. They, at least as much as we, can benefit from better understanding the advantages, as well as the potential pitfalls, of using the views of customers to help ensure the welfare of consumers.