There is a great amount of ongoing discussion about the need to develop new ways to assess and monitor a population's disease susceptibility to environmental factors. The ultimate goal in developing these tools, called environmental health indicators, is to increase the public health community's capacity for implementing interventions to prevent disease. Much of the discussion focuses on the requirement that the indicators be relatively easy and quick to apply. However, in the rush to find useful existing indicators, or to develop new ones, there is the danger that certain other important attributes of the indicator may be overlooked. These include: (a) whether the indicator truly represents an underlying causal relationship between an environmental exposure and a health consequence; and (b) whether the proposed indicator is a reasonably valid estimate of the underlying causal factor. This article provides a framework for relating environmental health indicators to the methods of epidemiology including some guidance for selecting and evaluating the appropriateness of proposed environmental health indicators. Examples are given which demonstrate how environmental health indicators can lead to a biased interpretation of underlying associations between environmental factors and the potential for disease when they are improperly conceived. These problems can be avoided by employing routine epidemiological concepts and methods as indicators are developed and evaluated.