The use of Langmuir or electric probes in collisionless plasmas which are of interest in sputter coating is briefly reviewed. The analysis of the probe current–voltage characteristic, especially that from double floating probes, to yield the plasma parameters, the electron temperature, and the ionization density, is considered in some detail. This paper is intended to be of use to the experimentalist who wishes to use Langmuir probes as a diagnostic tool but does not really want to become an expert in all aspects of probe theory and operation.