A Checklist for counsel to use to conduct a preliminary overlap assessment in a merger to identify areas of potential antitrust concern. This Checklist discusses how to identify the merging parties and their ultimate parent entities (UPEs) and their businesses, how to identify competitors, how to determine the size of the market, estimate market shares, and calculate market concentration using the Herfindahl-Hirschman Index (HHI), and how to analyze market entry.