Mass spectrometers are instruments capable of determining the composition of a gas and are one of the best ways to measure the isotope ratios of the constituents of a gas mixture. Mass spectrometers have been included on many major planetary missions and have provided key measurements for determining the composition, structure, and evolution of the planets and their atmospheres throughout the solar system. Several different kinds of mass spectrometers have explored the solar system. A description of the various types of mass spectrometers, an overview of the some of the planetary missions that have included mass spectrometers, and the results from those missions will be presented.