Determining the intrinsic microwave properties of materials is important for a variety of applications ranging from antenna and electronic circuit design to remote sensing to electromagnetic interference mitigation. A number of methods exist for characterizing intrinsic properties of materials at microwave frequencies, including transmission lines, resonant cavities, and impedance analysis. The use of free-space measurement methods has become commonplace among microwave material characterization laboratories due to its ease of use and reasonable accuracy. While some free-space facilities exist that can characterize down to 500 MHz, the method is most useful for characterizing materials from 2 GHz through millimeter waves. This book is designed to acquaint engineers and scientists with the theory and practice of using microwave focused beam systems for free-space characterization of materials.