This thesis presents three essays on the motives for mergers and the determinants of U.S. horizontal merger antitrust case selection. These essays contribute to the debate on whether mergers increase social welfare and on the efficiency of antitrust intervention.The first essay explores the market power motive for horizontal mergers by examining the relation between the announcement wealth effects to merging firms and their reliant corporate customers. Large sample studies generally conclude that efficiency considerations drive horizontal mergers and find little market power, which implies a non-negative wealth effect relation between these two parties along the supply chain. When I examine the endogenous stock market reactions to merger announcements with instrumentation, however, my results overturn this inference: I find that greater abnormal returns to merging firms systematically relate to lower abnormal returns to reliant customers. This wealth transfer effect exists for deals in industries with little foreign competition but not for deals in industries with intense foreign competition. These results suggest that increased market power is a key driver of horizontal mergers.In the second essay, I investigate the determinants of U.S. antitrust invention by examining horizontal merger antitrust case selection in the U.S. manufacturing sector during 1980-2009. I find no evidence supporting the consumer protection claim of the government's antitrust agencies. Instead, I find that the likelihood of antitrust intervention is negatively related to foreign import pressure. Hitting a market concentration hurdle criterion also predicts intervention. In addition, industry rivals seem able to exert pressure for antitrust intervention to avoid a competitive disadvantage. I identify two rival groups that account for the demand for antitrust regulation, local rivals and rivals producing less specialised products.The third essay examines the motives for related mergers from the perspective of product market similarity. Using Hoberg and Phillips' (2014) text-based product similarity measure, I find that when an acquirer's product is more similar to those of its rivals, a related merger results in a greater post-merger product price and lower market share for the combined firm. Moreover, for related mergers in more homogenous product markets, the stock market reactions to the merger announcement are higher for the combined firm and for product market rivals, but lower for reliant corporate customers. Overall, the evidence on both product market real performance and stock market reactions is consistent with the wealth transfer effect of related mergers, and suggests that the primary motive for firms to merge with product market competitors is to gain market power rather than to achieve efficiencies.