The past decade has witnessed remarkable developments in the quantitative analysis of horizontal mergers. Increases in computing power and the quantity and quality of data available have substantially reduced the costs of estimating demand systems using econometric methods. Good estimates of retail demand elasticities can make an important contribution to assessing the potential effects of a manufacturer merger on consumer prices. While estimates of demand relationships can make substantial contributions to merger analysis, it is much like every other area of empirical economics, in that practitioners invariably are forced to confront and resolve a series of difficult econometric and conceptual issues. The purpose of this paper is to identify a number of these issues that we believe researchers and practitioners should address, with the ultimate goal of improving the quality of antitrust analysis.