In this paper we review the results of optical emission spectroscopy (OES) in diagnostics of radio frequency (rf) discharges. The status of OES diagnostics of rf plasmas used primarily for plasma etching has been reviewed for the past 15 years. The results obtained at Keio University have been reviewed in greater detail. Time resolved and time-averaged computerized tomography (CT) of OES has been applied to obtain 2D and 3D profiles of emission for a large number of gases (Ar, H-2, SiH4, Cl-2, CF4, O-2, SF6) and gas mixtures. Application of OES-CT has become essential in understanding the sustaining mechanisms in capacitively coupled plasmas (CCPs) and in inductively coupled plasmas (ICPs) operating in rf. Those plasmas are produced in a wide range of frequencies, or combinations of frequencies and pulsed conditions. In addition, a wide range of geometries has been covered, providing an insight into spatial profiles of excitation (i.e. production of radicals) and therefore the uniformity of plasma etching. In our laboratory the technique was first applied to magnetrons. In CCP spatially resolved emission profiles were obtained for complex geometries and furthermore the effects of high frequency, two frequencies and pulsed operation were studied. Control of dust particles could be implemented by the same technique. Absolute values as well as the time resolved data showed the importance of the double layers in front of the instantaneous anode in maintaining the rf discharges in electro-negative gases. In ICP, CT was used to study the azimuthal anisotropy and the axial dependence of emission. Transition from capacitively to inductively coupled mode of operation could be proved directly by OES-CT profiles. Time resolved data for emission close to the coil showed two peaks with 45degrees phase difference, one of which was unexpected. In addition the effect of electro-negative gas and pulsing of the rf field were studied. It has been concluded that OES supplemented by fast, detailed models may be used for efficient real time control of plasma etching devices of the next generation. (C) 2002 Elsevier Science B.V. All rights reserved.