Regulations that affect hazardous-waste treatment, storage, and disposal have become increasingly stringent, leading to dramatic increases in the financial cost of legal waste disposal. Waste generators and haulers can respond to the changed conditions in several ways — by paying the increased costs of legal on- or off-site disposal; by reducing the amount of waste generated; by recycling; or by disposing illegally to air, water, or soil. A danger of policies that increase the costs of legal disposal methods is that firms may respond by diverting larger quantities of waste to illegal disposal routes. This study is a preliminary description of what is known about the extent and nature of illegal disposal, the types of firms that are involved, and the most promising enforcement strategies. The study is based on review of the available literature and interviews with approximately 40 enforcement personnel and industry representatives in Massachusetts, Pennsylvania, and Los Angeles County. The authors conclude that incentives to dispose of wastes illegally, and the frequency of illegal disposal, are likely to vary markedly among firms and to be correlated with observable attributes. In principle, it should be possible to target enforcement efforts by taking account of such factors. Unfortunately, systematic data needed to describe the quantity and nature of illegally disposed wastes, and to target enforcement efforts, are not available.