The contamination of epicontinentales waters, and therefore of the fish, historically has been documented in many regions of the world. The effort that is made to improve the quality of waters has obtained that the concentrations of some polluting agents are being reduced. Nevertheless, the concentrations in the aquatic ecosystems of polluting agents such as heavy metals, pesticides and toxic organic compounds have increased because the stability of them. The polluting agents come from allowed points of spill, accidental spills and sources of diffuse origin.