The state of the art of aquatic ecological risk assessment in the scientific and regulatory communities is reviewed. Historically, risk assessment has been conducted in response to environmental release of chemicals. Predictive capabilities have been developed and accepted only recently. Toxicity tests have changed little in the past four decades with acceptance by regulatory agencies coming many years after the tests are first introduced. Single-species laboratory toxicity tests using only a few designated species and a limited set of standardized testing methods form the basis of regulatory hazard evaluation. Recent scientific literature includes tests with alternative species and multispecies assemblages, but these tests have not been incorporated as of yet into the regulatory sequence of testing tiers for pesticides. The inclusion of the mesocosm test in the formal regulatory sequence at the highest tier represents a major step forward by the U.S. Environmental Protection Agency in that it requires testing on a much larger scale and with many more species than lower-tier tests required previously.