This book offers a comprehensive and timely review of planar microwave sensors based on coupled structures. Gathering chapters contributed by the most authoritative researchers on this topic, it presents various strategies for sensor performance optimization using coupled lines, directional couplers, and coupled resonators (either distributed or semi-lumped), focusing mainly on sensitivity optimization, and covering chains of coupled resonators as well. The book also reports on analytical methods, design formulas, sensor validation tests, with both simulation and experimental methods, describing some relevant practical applications. Mainly reflecting the research activity carried out by the different contributors in the last years, this book also includes two introductory chapters to help readers who are not very familiar with microwave sensing technologies and coupled structures. All in all, this book addresses advanced graduate students and researchers involved in microwave and sensor technologies, and may be of interest for engineers and professionals as well, working in areas as diverse as wireless sensors and sensor networks, biosensing, chemical sensing, motion control, microfluidics, Internet of Things (IoT), and smart systems.