Microplastics are increasingly detected in environmental and biological matrices, including water, soil, sediments, air, food, and living organisms. Their widespread occurrence has increased the need for reliable methods to identify and quantify different polymer types. FTIR and Raman spectroscopy are widely used for microplastic characterization, but their performance can be limited for very small particles, weathered plastics, complex mixtures, and samples rich in organic or inorganic matter. Pyrolysis-gas chromatography-mass spectrometry (Py-GC/MS) offers a complementary approach that can overcome some of these limitations. In Py-GC/MS, polymers are thermally decomposed into characteristic products, which are separated by gas chromatography and detected by mass spectrometry. These products can serve as diagnostic markers for polymers such as polyethylene (PE), polypropylene (PP), polystyrene (PS), polyvinyl chloride (PVC), and polyethylene terephthalate (PET). A key advantage of Py-GC/MS is its ability to provide mass-based quantification of individual polymers, including particles too small for reliable optical or spectroscopic analysis. However, as a destructive technique, it does not provide information on particle number, size, shape, or morphology. The reliability of Py-GC/MS depends on sample preparation, matrix composition, pyrolysis conditions, marker selection, instrumental parameters, and calibration strategies. Matrix components may generate interfering products, complicating polymer identification and quantification. In addition, the lack of harmonized procedures can limit comparability between studies and laboratories. Careful sample preparation, validated calibration, procedural blanks, reference materials, and quality-control measures are therefore essential. This review summarizes the principles and applications of Py-GC/MS for microplastic identification and quantification and discusses key analytical challenges, including sample preparation, polymer-specific marker selection, matrix interference, calibration, and method standardization.