In Gas Chromatography (GC), a microliter volume of sample is injected into a heated injection port, where the sample constituents immediately are evaporated. The GC oven can be operated in two different ways during the GC separation: in isothermal mode and in temperature programmed mode. In GC, two different types of columns are used: Capillary columns and packed columns. This chapter discusses that the GC-MS can provide important structural information of an unknown compound because interpretation of electron ionization (EI) mass spectra can provide information about the Molecular mass, Elemental composition, Empirical formula and Functional groups. It also discusses that GC-MS can be used for quantitative analysis. There are a large number of detectors available in GC, but only the standard detectors used in bioanalysis are described in the chapter. The chapter discusses the flame ionization detector (FID), the nitrogen-phosphorus detector (NPD), and the electron capture detector (ECD).