The paper deals with the theory and experimental development of resonator systems suitable for measurement of the permittivity and power factor of solid dielectric materials in the wavelength range below 50 cm (600 Mc/s). The relative suitability of three forms of resonator, namely a short-circuited length of coaxial transmission line operating in the principal mode, and hollow cylindrical cavity-resonators operating respectively in the E010and H01nmodes, is discussed. The theory governing the resonant behaviour of these systems when wholly and partially filled with “lossy” dielectric is developed, leading to relations connecting the permittivity and power factor of the latter with the resonant wavelength and Q-factor respectively.