One of the more highly researched topics in the financial economics literature has been the analysis of the gains made by shareholders of companies that participate in a merger and acquisition transaction. This paper surveys the evidence for which acquirer characteristic or merger transaction type generates non-positive or positive abnormal returns for the acquirer’s shareholders. In doing so, I describe a comprehensive set of hypotheses that has been built on existing theory, which has been tested using proxies variables in a regression specification. The chapter then explains that a number of new hypotheses have yet to be examined. Finally, the paper describes outstanding empirical issues in much of the existing literature.