This paper analyzes changes in concentration levels in the U.S. Advertising and Marketing Services (A&MS) industry using publicly released data that have been largely ignored in past discussions of the industrial organization of this industry, namely those available from the U.S. Census Bureau's quinquennial Economic Census and the Service Annual Survey. We define the A&MS industry in terms of nine sectors, each of which is represented by a separate 5 digit NAICS category. In so doing, we have sought to redress some of the measurement problems surrounding estimates found in the existing literature.