Instruments developed to measure the gas composition in vacuum systems are collectively referred to by the name residual gas analyzer, commonly abbreviated as RGA. Of the numerous RGA’s presented since the 1940’s, only the quadrupole mass spectrometer and autoresonant trap mass spectro­meter have developed into versatile residual gas analyzers over the years. Within vacuum technology the 180° magnetic deflection mass spectro­meter is still applied in helium leak detectors. In this chapter, the qualities, possibilities and limitations of these three mass spectrometers are discussed. Special attention is paid to their quality-determining parameter mass resolution, i.e. how distinct successive masses can be separately observed in the mass spectrum. Expressions for the theoretical resolution and its impact on the measured mass spectrum are derived.  Application of electron multipliers greatly increases the sensitivity of an RGA. Different types and their specific qualities are adressed. Finally, paragraphs are presented about the interpretation of residual gas spectra, how to perform spectrum analysis and some illustrative examples of mass spectra in vacuum systems.