Environmental assessment consists of the prediction of future changes in environmental quality and the valuation of these changes. The environmental assessment process has an objective component and a subjective component. The experience in less developed countries shows that environmental assessment procedures have promise, but have been of limited effectiveness to date. Mathematical modeling and simulation became feasible for use in environmental assessment with the greater availability of computers in the 1960s. Mathematical models are useful in environmental assessment because they allow internally consistent testing of alternative policies or projects. A complete environmental assessment method would ensure that environmental factors are included in an analysis, direct attention to alternatives, assist in quantifying impacts on a common basis, and communicate information to interested parties to facilitate the political process. Post-hoc audits after project implementation are desirable because they can test the impact predictions made in an environmental assessment, identify new problems, and suggest measures for mitigation or compensation of impacts.