IEC 61000-4-20 describes the use of transverse electromagnetic waveguides for both emission measurement and immunity testing of electrical and electronic equipment. A TEM waveguide, whether the conventional open-ended cell or the gigahertz transverse electromagnetic cell with its tapered structure and absorbing termination, creates a well defined electromagnetic field in a small volume, and therefore offers a compact and comparatively inexpensive alternative to an anechoic chamber. The standard explains the field structure inside the cell, the usable test volume in which the field is sufficiently uniform, and the field uniformity criteria that must be verified before a test is performed. It defines the methods for generating the field, for measuring and monitoring its strength, and for calculating the equivalent field that corresponds to a given input power. Both emission testing, in which the disturbance produced by the equipment is measured with a receiver connected to the cell, and immunity testing, in which a defined field is applied, are treated, and separate procedures are given for the conventional and the tapered cell because their field distributions differ.