So far in this book on radio-frequency plasmas the properties of plasmas have been investigated in the absence of periodic time-dependent parameters or boundary conditions, therefore effectively in a DC steady state. In this chapter the restriction to DC conditions will be relaxed to prepare the ground for the discussion of plasmas that are sustained by radio-frequency (RF) power supplies. Although quantities such as electric fields and potentials then become a combination of steady and periodic values, there are many useful situations that appear to be (RF) steady states when viewed over many cycles – all relevant quantities exhibit coherent oscillations and identical conditions are reproduced within each cycle. When the plasma is sustained by a combination of volume ionization and surface loss, and the response of ions is restricted by their inertia, as is the case in many RF plasmas, the density structure of the plasma shows barely any temporal modulation. The ion space charge in sheath regions is similarly robust. That is, the density profile of the plasma and that of the ions in the sheath remain steady. However, because the electrons are much more mobile, they are able to respond virtually instantaneously, thereby changing the spatial extent of sheaths and quasi-neutral plasmas. The potential profile is related to the spatial distribution of charges through Gauss's law, and this will change in line with applied potentials and consequent rapid redistribution of electrons.