Abstract: This review of the literature on neoplastic or cancer-like disorders in fish and shellfish sets out to (1) summarize what is known of the occurrence of neoplastic diseases in indigenous populations of aquatic animals found in different geographic areas of the world; (2) critically evaluate existing reports so as to determine whether or not environmental pollutants are implicated as causal agents of the cellular disorders and (3) describe the current state of knowledge in the general areas of fish and shellfish metabolism and the use of aquatic animal models in studies of chemical carcinogenesis. Tumors and neoplasms have been described in fish and shellfish populations from many areas throughout the world. Although environmental pollutants have been suggested as the cause of neoplasms in at least some cases, the existing evidence does not permit a firm conclusion. For fish, the quality of the reports and data reviewed varied considerably. There are rather good experimental data to suggest that cancer-like conditions in fish from certain areas of Puget Sound (Washington), the Fox River (Illinois) and Japan are associated with chemical contaminants in the environment. The data purported to support a chemical etiology for highly publicized tumors in fish from the Buffalo River, Torch Lake and Black River are not of a high quality. Most of the studies from those areas are compromised by inadequate experimental designs, and further research will be necessary to achieve a more complete in relation to the existence of tumors in fish from those areas. There are also reports providing impressive evidence that chemical pollutants are not associated with neoplasms in fish, even in those inhibiting environments known to contain mutagenic substances. Large surveys conducted from highly polluted aquatic systems in Yugoslavia (Sava River), Germany (Rhine and Elbe Rivers) and Australia (Port Phillip Bay) all reported negative results. Many other studies and surveys have produced data that neither supported nor refuted a pollution-neoplasm association. In contrast to fish, there is little evidence that neoplasms of clams, oysters and mussels are associated in any significant way with environmental pollution. The most obvious conclusion, from reviewing the existing literature, is that much additional research is required before the relationship(s) between environmental pollution and neoplasms in indigenous aquatic species can be understood. There are severe constraints associated with conducting credible field studies in this area, given the incalculable number of variables in natural environments. In the future, it may be advisable to place more emphasis on laboratory studies employing state-of-the-art methods to provide fundamental information that can be used to understand the nature and causes of neoplasms in aquatic species.