In recent years, the FTC has achieved significant victories in health care merger cases based on traditional geographic market analysis under the Merger Guidelines. However, the FTC appears to be contemplating a new enforcement frontier that stretches into an area beyond the standard Guidelines analyses: “cross market” transactions between providers in different geographic markets. This theory has the potential to expand significantly the scope of mergers subject to increased antitrust scrutiny. It is not apparent, however, that the theory or empirical analysis have reached the level to justify expanding merger reviews. The article describes the development of cross-market merger theory and examines the emerging economic literature.