At least 17 pesticides have been found in ground water in a total of 23 states as a result of agricultural practice. These results have been obtained through three different types of monitoring studies: large-scale retrospective, small-scale retrospective, and small-scale prospective. The first two types of studies survey areas where the pesticide(s) in question has already been used. The third type of study is an intensive field study where the pesticide is applied and monitoring begins at time zero. Often, soil core data are at least as important as ground water data. The ability to draw meaningful conclusions from large-scale studies is greatly diminished unless the studies have a statistical, stratified design. The purpose of this paper is threefold: to describe the three study types; suggest guidelines for groundwater sampling, soil sampling and well construction; and update the data summary of pesticides in ground water from agricultural practice.