Under certain conditions EPA requires simulated or actual field testing for the registration of pesticides. Simulated field tests are generally known as mesocosm studies, while farm pond studies are field tests performed under actual pesticide use conditions. Both approaches have advantages and disadvantages. Pesticide application to mesocosms is based on computer simulations. In farm pond studies, chemicals are applied to fields surrounding ponds and follow natural routes of entry into the aquatic environment. In mesocosm studies, simple pond-like systems at an early stage of ecological succession are replicated at one location enabling a statistical evaluation. Farm ponds are often located over a larger area making true replication difficult. However, as farm ponds are relatively stable, changes in ecosystem composition or function can be observed even within one system. Biological sampling is similar in both methods, but the agricultural field portion of a farm pond study adds efforts in sampling and chemical analyses. Depending on the specific objective of the study, both methods can yield valuable results. As ecosystem studies require considerable efforts the problem should be defined clearly, so the results can be used for a firm decision.