Editors' Summary: The Environmental Protection Agency's (EPA's) regulatory definition of "solid waste" in the Resource Conservation and Recovery Act (RCRA) has long been a source of controversy. Escape from the definition of "solid waste" and in most cases you have escaped from regulation under RCRA. To a large extent, in defining "solid waste" EPA is faced with often conflicting goals embedded in RCRA itself: RCRA regulates wastes, but "non-wastes" are frequently just as hazardous to humans and the environment. Congress' choice to regulate only wastes under RCRA was a calculated risk: trying to comprehensively regulate all hazardous materials would involve enormous compliance costs for industry and mammoth federal bureaucracy. At the same time, regulating only "wastes" creates strong incentives to label dangerous chemicals "non-wastes" whenever possible. In defining "solid waste," EPA must make the definition comprehensive enough to include all wastes, but not so comprehensive that non-wastes are inadvertently included also. In this Dialogue, the author summarizes the regulatory definition of "solid waste" and explores some of the ambiguities it presents. He observes that a set of questions arises from the attempt to set forth broad categories of wastes and activities, while in practicemany cases will not fit cleanly into any of these categories. Moreover, he focuses on the regulatory distinctions between by-products and co-products, as well as conflicting guidance found within the regulation itself and its accompanying explanatory document.