The 1999 acquisition of Caesars World, Inc. by Park Place Entertainment Corporation generated concerns for increased and undue economic concentration in the Atlantic City market, although less concern has been expressed for increased concentration in national and international markets. This article explores the economic concentration effects of this acquisition in the Atlantic City market and the opposing effects of proposed entry of from one to three casino/hotels. The effects are measured by changes in the Herfindahl-Hirschman Index of market concentration. This index is the measure of economic concentration historically used in hearings before the New Jersey Casino Control Commission whose charge it is to disallow “undue economic concentration.” Several issues relevant to acquisition, entry, and concentration are discussed. These include the likelihood and time delay of entry, the decline in concentration given a wider geographic market, rivalry, economies of scale, contestable markets, acquisition justified by financial weakness, Federal Trade Commission guidelines, and concentration in the Las Vegas market.