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The biological monitoring based on the community structure or the presence of selected species or groups of species may supply a synthetic, direct and integrated response over time of the pollution level of the environment. The most important disadvantage of this type of biomonitoring consists in the impossibility to identify the nature of the pollutants. This may be overcome by analyzing the pollutants in the tissues of selected species called ‘accumulator organisms’ or ‘scavengers’ which are able to concentrate pollutants even from very diluted solutions. This technique initially used to detect radionuclides from fall-out and nuclear effluents, has been successfully applied to non-nuclear pollution. This method is particularly useful, (1) to map the pollutant distribution in an area or along a gradient; (2) to follow the variations of the pollutant concentrations in the environment by the variations of the pollutant in the organism and (3) to identify toxic substances from solutions so diluted that they cannot be detected by the commonly used analytical method. The criteria and methods adopted by this type of monitoring are discussed and illustrated by selected examples.