Collusion by participants often poses a serious threat to markets and organizations. Nowhere is such a threat more intimately felt than in auctions where bidders can manipulate or simply withdraw their bids to limit competition. Not surprisingly, auctions have provided the volume and prominence to the study of collusion, with its evidence found in highway construction contracts (Porter and Zona (1993)), timber sales (Baldwin et al. (1997)), and in school milk delivery contracts (Pesendorfer (2000), and Porter and Zona (1999)).