Molecular Beam Mass Spectrometry is a very versatile diagnostic for the investigation of plasmas. Utilizing a three-stage differentially pumped system allows to measure stable gas phase products, as well as reactive species and determine their densities. A collision-less transport of the process gas from atmospheric pressure into high vacuum allows the analysis with a quadrupole mass spectrometer. The measurement of species over a wide mass range directly from the ground state is possible without the need of suitable optical transitions. The implementation of a multichannel scaler card further allows a very high time resolution.