Theory and measurements of the conductivity and the (transient) photoconductivity in the microwave frequency range are presented. The theory is tested on noninvasive measurements of semiconductors with known properties, i.e., Si wafers, in a simple apparatus. Quantitative agreement between theory and experiment is found without the use of adjustable parameters. A contactless and accurate determination of the conductivity of Si wafers in a restricted conductivity range is proposed. The quantitative evaluation of photoconductivity measurements makes a detailed discussion of nonuniform photoconductivity possible. The requirements for reliable measurements of nonhomogeneous charge carrier kinetics are discussed. ©1995 American Institute of Physics.