Assessment of impact can have at least three meanings: the evaluation, prior to impact, of the loss or alteration of a natural community; the prediction of impact; and the measurement of the extent of impact after it has occurred. Measurement of impact in environmental impact assessment (EIA) for aquatic biota is relatively rare. In most cases, scientists are required to predict impacts without testing whether these predictions hold following development. There are numerous study approaches in EIA that form the basis for predictions. These include a reliance on existing information, snapshot surveys, and surveys conducted over several sampling periods prior to impact. The latter two approaches can be divided into surveys of the impact site alone or surveys of the impact site plus control sites. There also is an issue-oriented approach in which the description of the environment is more closely linked to the predicted physical effects of the proposal. An example is given in which organisms at existing marinas are compared with those at natural sites in an area where a new marina is proposed. While there may be a sound scientific basis for the selection of a particular study approach, there are other considerations such as the cost of the proposal, the legislative framework for EIA, the scale of the proposal, and the attitude of the proponent. It is important that predictions of impact be stated as clearly as possible so that they can be evaluated readily and can facilitate monitoring if required. An area in which monitoring could be of value for improving the accuracy of EIA predictions is in small-scale developments, such as small marinas and maintenance dredging.