Gas sampling is still widely used for analyzing chemical composition. As the phrase gas sampling suggests, a volume of gas is sampled (usually continuously) from the flow and sent through analyzers to discern the various gas molecules present in the sample. Gas sampling analyzers represent a mature technology, can be calibrated using reference gases, reach sensitivities down to ppm levels, and are rugged and readily available from various vendors. There is a wide variety of analyzers for detection of pollutant and toxic gases, fuel and oxidizers, and for identifying a complete spectrum of molecules (gas chromatography and mass spectroscopy). In many chemically reacting flows, including engine combustion, power plants, environmental monitoring and chemical processing, it is of interest to determine the constituents of gas going in and out of systems for efficiency, process control, and pollutant emissions. For these reasons, gas analyzers represent an important diagnostic method for direct monitoring of chemical composition or calibration of other sensor devices. On the other hand, the sampling probes do introduce fluid dynamic perturbations to the flow and also suffer from relatively poor spatial resolutions. The analyzers are often bulky and heavy, and sampling requires care to preserve the content of the gas from the sampling point to the analyzer. In this chapter, we will begin by discussing the considerations for accurate physical sampling of gas using probes and go on to present various analysis methods for detection and monitoring of gas species concentrations. Figure 7.1 shows a general schematic of a gas-sampling probe connected to a gas analyzer. A pump draws the sample from the test section through the sampling probe and the analyzer. To optimize the sampling condition at the probe, pressures are monitored and controlled in the sampling line.