Environmental fate and effects studies of chemical contaminants conducted in marine and estuarine microcosm and mesocosm test systems are reviewed. Contaminant fate, persistence, and transport are discussed and related to exposure regimes that result in effects on marine biota. The value of these systems in providing controlled environments for studying contaminant issues at the individual, population, community, and system level of assessment lies in the ability to interrelate toxic effects at one level of assessment to changes observed at another. Examples of contaminant studies in microcosm and mesocosm test systems are presented to illustrate various approaches for ecotoxicological studies. Field calibration and validation of test systems are reviewed and example projects discussed.