The complexity and interrelationships of human environments are constantly undergoing further definition. The importance of such definition can be realized when such broad environmental categories as natural ecosystems, the agroecosystem, occupational, urban, domestic, and military deployments, all of which can be subdivided into more specialized environments, are considered. Set against this background, it is clear that the principal role of toxicology is risk analysis. Risk analysis consists not only of risk assessment but also risk management and risk communication. New paradigms in hazard assessment are revolutionizing risk assessment, and all aspects, from molecular toxicology to outreach programs, will be involved in future considerations of human environments.