The application of GC and MS techniques is documented with the results from studies on chlorinated contaminants in environmental and biological systems. Among the compounds investigated were DDT and related compounds, chlordane and toxaphene, hexachlorocyclohexanes (HCHs), polychlorobiphenyls (PCBs), -terphenyls (PCTs) and -naphthalenes (PCNs), and polychlorodibenzodioxins (PCDDs) and -dibenzofurans (PCDFs). The residues result from applications of these chemicals as pesticides (HCHs, DDT, chlordane and toxaphene), as industrial chemicals (PCBs, chlorophenols) or as a result of technological processes such as incineration (PCDDs, PCDFs), and bioaccumulation of these compounds in the environment. Some of these series consist of a large number of individual components with various chloro congeners and isomers. Some of these contaminants are chiral and exist as enantiomers with different biological properties (a-HCH, 2,4'-DDT, several chlordane components and metabolites, most toxaphene components, certain PCBs). However, the chirality of some of these contaminants has only relatively recently attained some attention. MS plays a dominating role in the analysis of all these compounds, usually in combination with high-resolution gas chromatography (HRGC) for the detection at trace level concentrations (ppb- to ppt range). Chiral HRGC is used for the enantioselective determination of chiral contaminants. In this case, different cyclodextrin derivatives are added to the GC stationary phases as chiral selectors. Both, El and electron-capture, negative-ion (ECNI) MS are extensively used. Selected-ion-monitoring (SIM) techniques are used for detection of target compounds and include the use of stable-isotope-labeled materials such as the ¹³C-analogs of these contaminants for quantitation. Full-scan MS is used for confirmatory purposes and for identification of unknown components. ECNI MS is highly sensitive for higher chlorinated congeners such as for toxaphene. It allowed a distinction among chloro homolog groups but no further differentiation among isomers. EI MS showed more fragmentation with large differences among isomers. The monitoring of specific ion transitions (RDA fragmentation pathways) in EI MS/MS experiments allowed isomer-specific, and in combination with chiral HRGC, enantioselective analyses. The application of these techniques to environmental and biological samples is illustrated, particularily to those of aquatic species including some from the Arctic and Antarctic. The result of several chiral contaminants showed largely deviating enantiomeric compositions in environmental samples, deviating from the usual 1:1 composition in the technical products. These changes can only be the results of enantioselective biological processes (metabolism) and cannot result from abiotic processes (chemical, photochemical, distribution, transport) in the environment.