Numerous diagnostics have been used to help characterize and understand conventional magnetron plasmas. These include Langmuir probe studies, magnetic measurements of the drift currents, optical measurements of the sheath, along with emission spectroscopy of the sputtered species. The plasma conductivity is described by Bohm diffusion, and the sheaths by Child's law. Local gas density measurements in the near-cathode region show significant reductions in the gas density as a function of discharge current. The plasma in a magnetron is strongly perturbed by the energetic sputtered species. The plasma impedance as a function of current is significantly altered by the magnitude and energy of the sputtered species, as well as the gas cross section and velocity. This effect has been modeled in terms of the energetic sputtered atom flux into the plasma region and the thermal conductivity of the gas. Deposition measurements on the cathode itself document the effects of gas scattering and the density modification due to the heating by the sputtered flux. The gas rarefaction effect modifies the energetic particle bombardment of the film in a nonlinear way with rate. It modifies the effective energy transferred to a film with each depositing atom, and alters the transition of elemental targets to the compound form when sputtering in a reactive atmosphere. © 1988, American Vacuum Society. All rights reserved.