There are well over 500 registered pesticides worldwide for use in agricultural regions and new agrochemicals are introduced to the marketplace continuously. This chapter deals with the chemical analysis methods for the main pesticide chemical classes that are most frequently analyzed with gas chromatography (GC) or liquid chromatography (LC) coupled to mass spectrometry (MS). GC amenable pesticide chemical classes which do not require derivatization include organochlorines (OCs), pyrethroids, organophosphorus pesticides (OPs), triazines, and chloroacetanilides. In addition some transformation products of organochlorines, triazines, and phenylureas are GC amenable and when derivatized some transformation products of OPs, pyrethroids, and phenoxyacid herbicides are also GC amenable. Specific methods have been developed with other injector choices than the standard splitless injection for more thermally labile chemical classes such as trihalomethylthio fungicides to extend the range of GC amenable pesticides. Some chemical classes which are more polar such as phenoxy acid herbicides and carbamates can still be analyzed by GC/MS methods but require derivatization to make them GC amenable. For some other chemical classes a few pesticides have been analyzed by GC/MS usually included in multiresidue methods but these methods have not tackled the entire range of compounds within the chemical class. These include chemical classes such as dicarboximides (vinclozin, iprodione), dinitroaniline (trifluralin, ethalfluralin), dinitrophenol (dinoseb), and dithiocarbamate (triallate). A large number of pesticide classes generally of higher polarity suffer from poor chromatographic performance, poor MS source ionization or stability in GC/MS injectors, on-column, or in MS. For these chemical classes and also to minimize the need for derivatization prior to GC there has been a gradual shift to the development of new methods utilizing LC coupled with tandem mass spectrometry (MS/MS). Tandem mass spectrometry in selected reaction monitoring (SRM) mode is generally now more frequently used for LC rather than selected ion monitoring (SIM) with LC/MS as the ionization process for LC/MS is a softer process (change processes to process) than that of GC/MS ion sources such as EI and CI. For atmospheric pressure ionization (API) sources most frequently used in LC/MS/MS most pesticides have only one ion formed during ionization (the protonated or deprotonated molecular ion or sometimes an adduct ion (eg. sodium or ammonium adduct)) and consequently there is little confirmation ability. Tandem mass spectrometry allows for the controlled collision induced dissociation