Identification of persistent organic pollutants (POPs) remains one of the most challenging aspects of environmental chemistry. This is partly the result of a dearth of toxicological data, genuine analytical standards, adequate chemical libraries and analytical methodologies necessary to distinguish suspected POPs from among the c. 100,000 chemicals used in industry and commerce. One approach is to tailor detection methods such that POPs can be recognised on the basis of similar chemical properties and structure motifs without specific a priori knowledge of their identities. For example, the majority of known POPs are halogenated. Mass spectrometry coupled to (multidimensional) gas chromatography is ideally suited to differentiate halogenated compounds from nonhalogenated compounds on the basis of isotope pattern, mass defect and chromatographic separation. This Chapter describes various techniques and strategies that employ time-of-flight mass spectrometry, comprehensive two-dimensional gas chromatography, GC×GC, and Fourier transform mass spectrometry for the nontargeted identification of halogenated POPs.