Market structure is an essential topic in economics and finance courses, including bank management as well as many other business school courses, for example marketing, human resources and strategic management. Instructors explain the virtues of perfect competition and the evils of monopoly along with alternative market models. Often conversations drift into discussing mergers and acquisitions, specifically why some mergers are approved while others are denied. This paper provides instructors with a handy resource that assists students in understanding changes in market concentration using "real-world" banking examples and "what-if" scenarios. It also introduces the recently revised Department of Justice/Federal Trade Commission Horizontal Merger Guidelines. Finally the paper discusses several issues closely related to market concentration.