Fixed and variable length re‐entrant resonant cavities designed for the measurement of dielectric constant and dissipation factor are described. These cavities operate in the frequency decade of 108to 109cycles per second, a region avoided by many experimenters because the frequency is too high for the application of circuit techniques and not high enough for the convenient use of coaxial lines or wave guides. The theoretical considerations in the design of these cavities are presented. The well‐known susceptance variation method, widely used in the frequency range of 104to 108cycles per second, was extended to apply to these cavities; it yields a rapid measuring technique and very simple expressions for calculating the values of the dielectric properties. The performance of this apparatus is discussed and the results of the measurements of a few typical dielectrics are given.