Open resonator microwave sensors allow accurate sensing, monitoring and measurement of properties such as dimension and moisture content in materials including dielectrics, rubber, polymers, paper, fabrics and wood veneers. This book presents a coherent and entirely practical approach to the design and use of systems based on these sensors in industrial environments, showing how they can provide meaningful, accurate and industrially-viable methods of gauging. Starting with an introduction to the underlying theory, the book proceeds through the entire design process, including simulation, experimentation, prototyping and testing of a complete system. It takes the reader through the development of a particular sensor, stressing the parameters that should be optimized and emphasizing practical aspects of a sensor and of its use. Two extended application case studies on the use of these systems for rubber thickness and fabric coating monitoring are included.