Collusion among buyers leads to social welfare losses, which provide the economic rationale for public enforcement of the antitrust law. This conduct also imposes losses on the victimized sellers, which provide the foundation for private enforcement through private damage actions. In this paper, we present a rigorous economic analysis of buyer cartels. This effort includes both full participation and partial conspiracies. We review the antitrust treatment of collusive monopsony in the United States, the European Union, and Asia, offer a measure of antitrust damages, and examine the necessary precision of the damage estimate. We also suggest that the proper use of modern econometrics should allay judicial concerns with speculation.