Low pressure, radio-frequency (rf) glow discharges have been used extensively in plasma processing in recent years. To exploit the advantages of the rf plasma fully, a detailed knowledge of the discharge physics is required. Many powerful models have evolved which attempt to describe the plasma and predict plasma parameters under varying conditions. However, in the absence of versatile and reliable diagnostic experimental tools, such models cannot be tested and the processing operator must rely on the so-called "Response Surface Methodology" to predict plasma-chemistry and physical reaction rates. This thesis reports on the development of a tuned Langmuir probe, which has been used to monitor plasma parameters, including the electron energy distribution function (eedf), in 13.56MHz argon and nitrogen plasmas under various discharge conditions. The development and application of the tuned probe is described, as is the rf environment, since successful measurements rely on a thorough understanding of the subtleties of the two. The plasma parameter measurements are presented as a function of gas pressure, spatial distribution and input rf power. The results are generally interpreted in terms of accepted rf plasma theory, which is still evolving. The eedf measurements demonstrate that the argon and nitrogen plasmas are created via different ionization processes, and may be strongly influenced by inelastic and super-elastic collision processes. Plasma heating, a topic of much recent debate, is discussed, and an electron trapping mechanism, which resolves some of the questions posed by the eedf measurements and the current plasma heating theories, is presented.