The structure and function of fish gills are briefly summarized, and the responses to the environmental pollutants are reviewed. Some of environmental pollutants, such as acid rain, metals and organic xenobiotics cause morphological and physiological changes in the gill. The toxicities of these environmental pollutants are often influenced by the conditions of surrounding water, such as pH, hardness, salinity, etc. Therefore, these information will be helpful for evaluating aquatic ecological effects of environmental pollutants.