The effects of excitation frequency on electrical discharges used for low pressure plasma processing are discussed and illustrated with examples. Frequency alters the spatial distribution of species and electrical fields across the discharge, the energies and concentrations of species as a function of time, the characteristic potentials and energy of ions impinging on surfaces, and the electron energy distribution function. Important transitions in the behavior of these quantities occur when the excitation frequency crosses the ion-sheath transit frequency (ITF), the electron energy modulation frequency (νu), the electron momentum collision frequency (ν), and the plasma frequency (ωp).