Mixture models were studied in an effort to predict the microwave frequency permittivities of unidirectional-fiber-reinforced thermoplastic-matrix composite materials as a function of fiber volume fraction, fiber orientation relative to the electric field, and temperature. The permittivities of the constituent fiber and plastic materials were measured using a resonant cavity perturbation technique at 9.4 GHz and 2.45 GHz. The permittivities of the composite specimens were measured using a reflection cavity technique at 9.4 GHz and 2.45 GHz. Simple" rule of -mixtures II models that use the fiber and plastic permittivities have been found to approximate the complex dielectric properties of the composite for varied fiber volume fractions. The permittivities of oriented composites were successfully modeled at 9.4 GHz using a tensor rotation procedure. Composite permittivities were modeled with temperature up to the glass transition temperature of the thermoplastic matrix. Good agreement was found between the mixture model and experimental results for permittivity as a function of temperature at 9.4 GHz. / Master of Science