Radio frequency generated plasmas are used extensively in commercial manufacturing systems for a range of different applications including reactive ion etching, thin film deposition and surface modification. To optimise such processes a thorough understanding of the physical processes within the plasma is essential. In this thesis low pressure radio frequency argon plasmas are investigated in order to understand further the physics of their behaviour under certain conditions. Tuned Langmuir probes and microwave mterferometry are used as diagnostics methods to measure different plasma parameters as a function of gas pressure, current density and applied voltage. The spatial variations of these parameters are examined within the bulk plasma. In particular the electron distribution function was obtained from the Langmuir probe to observe transitions between heating modes which occur under certain conditions of the plasma. To further understand this transition a small variable transverse magnetic field was applied to the plasma to confine the electrons and modify their trajectories Mathematical models devised and compared to the experimental results showed an excellent agreement. Finally the tuned probe was installed on other research systems in Ireland and abroad to characterise the plasma produced by different gases, with other methods of power coupling and in various chamber geometries.