Of the thousands of species of microalgae that form the base of the marine food chain, only a small number are toxic or harmful. However, when these toxic species proliferate, they can cause massive kills of fish and shellfish, mortality among marine mammals and seabirds, substantive alterations of marine habitats, and human illness and death. Currently, six distinct human clinical syndromes associated with harmful algal blooms are recognized: ciguatera fish poisoning, paralytic shellfish poisoning, neurotoxic shellfish poisoning, diarrhetic shellfish poisoning, amnesic shellfish poisoning, and Pfiesteria-associated syndrome. Human illnesses are caused by toxins produced by these microorganisms, acquired either by passage through the food chain or direct skin or respiratory contact. Syndromes frequently include debilitating neurologic manifestations and, in some instances, may progress to death. There is a perception among investigators that the number of harmful algal blooms is increasing, as is the range of toxic species. It has been postulated that this increase is caused by human-related phenomena such as disruption of ecosystems, nutrient enrichment of waterways, and climatic change. In environmental studies, attention has traditionally focused on direct human health effects of pollutants. Harmful algal blooms are an example of an alternative paradigm, in which human-induced stress on complex ecologic systems leads to the emergence of new, potentially harmful microorganisms (or the reemergence of "old" pathogens from previously restricted environmental niches), which, in turn, cause human disease. Although data are lacking to fully substantiate this latter model, it provides a useful conceptual framework to assess data needs and consider public health interventions.