Since 1975, the Office of Pesticide Programs (OPP) has included aquatic field testing in its aquatic tier testing scheme. Beginning in 1988, OPP provided additional guidance concerning such testing and focused on mesocosms as the preferred method for aquatic field testing. Using endpoints as described in the regulations, and considering the weight-of-evidence approach to ecological risk assessment, we will describe why mesocosms are important and how mesocosms are used in OPP’s ecological risk assessment process.