The role of underwater sound as a potential stressor in the marine environment is now widely recognised and the designers of sonars find themselves increasingly constrained by environmental legislation which requires them to consider the possible harmful effects of high power sound transmissions on marine life (e.g. fish and marine mammals) and on human beings. This paper describes a formal process of environmental impact assessment being developed in support of the procurement of future sonars in the UK. The basis of this process in environmental legislation is briefly reviewed but the main purpose of the paper will be to consider the complex scientific and technical issues surrounding environmental impact assessment. In particular, Environmental Assessment (EA) for sonar systems requires a process of cause and affect modelling to be undertaken. Sonars may produce both energy and substance pollution (e.g. explosives may release toxic compounds). The 'precautionary principle' which is enshrined in environmental legislation puts the onus on the polluter to prove that his particular form of pollution does not have a harmful effect on the environment. But how are environmental impact criteria defined? Toxic effects are relatively easy to test for and to quantify. Sound energy on the other hand is rather more difficult. Consideration of the hearing sensitivity of fish, for example, leads to the notion of safe exposure level and probability of avoidance. But how representative are the experiments on which these criteria are based (e.g. the impact of seismic airguns on fish catch rates)? How can we assess the reliability of the scientific evidence given the uncertainties elsewhere, e.g. poor or inadequate knowledge of sound propagation characteristics (including non-linear effects associated with impulsive sound sources), uncertainty in environmental conditions, natural variability and the cumulative effects of repeated exposure to sound energy transmissions? There have been few coincident measurements of sound intensity in the ocean at the ranges at which a particular species exhibits avoidance behaviour and many studies make simplifying assumptions regarding acoustic propagation, e.g. spherical spreading out to unrealistically long ranges, These and other topics will be reviewed with example calculations 'to illustrate particular aspects of the EA process which is being developed by the authors.