This title provides a unique theoretical framework for multi-mode resonant antennas and different approaches to their implementation, with an emphasis on mode gauge functionality, a new concept for a clear identification and flexible control of all usable resonant modes in multi-mode resonant antenna design. The book commences by advancing a generalized odd-even mode theory as a general theoretical framework for resonant elementary antennas, offering new insights into the classical problem of coupling effects between antenna and transmission lines and helping reveal the operation mechanism of elementary antennas under multi-mode resonance. Then, the concept of "mode gauge" is developed and employed for wideband elementary antenna design by simultaneously exciting and tuning multiple resonant modes within a single radiator. Apart from theoretical explorations, the authors also provide analysis of up-to-date implementation of multi-mode resonant elementary antennas with different functionalities, including wideband antennas, circularly polarized antennas, multiband antennas, frequency scanning antennas and low-profile antennas. Academics, students and professional engineers at all levels will greatly benefit from the book and will be provided with historical background, state-of-the-art methodology, useful design tools and multiple applications of multi-mode resonant antennas.