Environmental pollutants can cause many different types of histopathological changes in marine bivalves. By using routine histological techniques and the light microscope, these changes can be observed. Among these changes are an increase in the number of concretions in quahog kidneys, gonadal tumors in quahogs and soft shell clams, loss of cilia from the gills of blue mussels, and blood vessel abnormalities in oysters exposed to arsenic. These and many other lesions found in pollution stressed animals indicate that histopathology is a valuable tool for monitoring the environment.