Multifunctional Adaptive Microwave Circuits and Systems is a state-of-the-art survey on Multi-functional, Adaptive Radio Radar and Sensor, or MARRS, systems and summarizes the findings of a five year project entitled MARRS: Multifunctional Adaptive Radio Radar and Sensors that ran from 2001 to 2006. The book presents a systematic approach to the system level design required to develop MARRS technology. Developments of the most viable tunable and reconfigurable components are described. These are Micro Electro-Mechanical System (MEMS) switches and varactors. The book describes electrically-tunable varactors made from ferroelectric materials. Applications in tunable switches, tunable filters, tunable matching networks, and tunable antennas are presented. Concepts and architectures for RF transmitters designed to optimize performance by adapting to variable loads, power levels, and linearity require-ments are explored, as well as reconfigurable and tunable RF filters using both semiconductor and ferroelectric varactors. The book also discusses retro-directive antenna arrays, Finally, a multifunction radar, communication and radiometry system that utilizes a retro-directive antenna array is described.