The use of various whole-cell organisms as tools for monitoring water contaminants is reviewed and evaluated. Their suitability and value for assessing a range of environmental problems from the impact of pollution, homeland defense issues, conservation, and the long-term degradation and recovery of ecosystems is discussed. Guidelines are provided for the choice of appropriate bioreporter organisms, transducers, and immobilization methods. Examples of the use of a broad spectrum of whole-cell bioreporters in assessing a variety of environmental problems are summarized.