Work on indicators is critically reviewed, focusing on a number of key issues ; air and water pollution ; natural resources and biodiversity ; climate change ; ozone depletion ; public health; demography ; production ; consumption ; and technology ; waste ; and costs of environmental protection measures. Indicators have an added significance as compared to the underlying statistics and are tied to a specific purpose. For simplicity the number of indicators can be reduced by aggregating them into an index. A conceptual framework is proposed for the selection, development and analysis of indicators. This framework comprises three interrelated sub-systems: the human population ; production, consumption and technology ; and the environment. An integrated analyses of the state of the environment requires indicators that capture not only the quality of the environment, but also the forces that change the environment, and the responses to these changes by human society. These crucial interactions can be better understood if a conceptual or computational modelling framework is developed and used. The characteristics of indicator systems have been determined mainly by the producers of the information rather than by its users. Furthermore, the present sets of environmental indicators have been mainly selected by the industrialized countries, for their own needs. Indicators should be developed according to their envisaged applications. These applications include early warning and evaluation of policy performance. As countries develop, priorities shift over time, requiring adjustments of indicator systems. The reliability of indicators is mainly determined by the reliability of the underlying statistics and raw data. Harmonization of definitions and measurements methods and the development of standards should be further pursued.