This chapter presents general methodology for the analysis of organophosphorus pesticides. Methods specific for water analysis will be discussed and a suitable multiresidue method will be detailed. As yet, there is no multi-residue method designed specifically for the analysis of organophosphorus pesticides in sediment and fish samples; however, some discussion on the analysis of these substrates will be included based on published methods for soil, plants, and animal tissue. The instability or short half-life of an organophosphorus pesticide after application is beneficial to the environment, but it also causes analytical problems. For analytical data to be valid, they should reflect the concentration of the pesticide at the time of sampling. Paper chromatography is simple and relatively cheap for use in organophosphorus pesticide analysis. An aqueous and/or nonaqueous solvent developing system may be used with either 1- or 2-dimensional paper chromatography.