The potential impact of the environmental pollutants on human health can be evaluated by the laboratory analysis of the environmental samples or by the measurement of the biological effects on indigenous populations and/or specific test organisms placed in the environment to be monitored. A canary in a cage, used by 19th century miners as a biological indicator for rising levels of toxic gases, is a classical example of in situ hazard identification. The induced toxic effects are often the result of synergistic and antagonistic interactions among various physical and chemical factors that are difficult to reproduce in the laboratory. Therefore, conceivably the biological effects measured on or near the impacted site have greater relevancy for hazard assessment to man than from the data derived from the environmental samples analyzed in the lab. The organisms most commonly employed for the assessment of mutagenicity under real-world conditions are: (1) flowering plants, (2) wild and captive mammals, and (3) aquatic vertebrates. Plant species such as Tradescantia paludosa, Zea mays, and Osmunda regalis have been used for monitoring ambient air quality around several major industrial cities in the U.S.A., nuclear power plants, and industrial waste sites, and also for the assessment of potential health effects of municipal sewage sludges. Domestic animals such as dogs can be used as sentinels to provide information on the effects of contaminants in the environment and have been used to a limited extent to evaluate the environmental influences on the occurrence of breast cancer and osteosarcoma. Cytogenetic analysis from feral and wild animals has been employed for assessing the health hazards and prioritizing the clean-up efforts at hazardous waste sites. Aquatic animals have been used more often than terrestrial animals or plants to identify and characterize the genotoxic effects of environmental pollution. Since 1970, a number of studies has been reported on the mutagenic and neoplastic effects on aquatic animals from coastal areas and continental rivers, lakes, and ponds. The limitations of in situ environmental assessment are lack of control over the physical environmental components, inherent variability and interactions of test organisms, lack of control of exposure doses, and difficulty of finding concurrent experimental controls. Nevertheless, flowering plants, terrestrial, and aquatic animals may serve as useful sentinels and biomarkers of environmental pollution.