Abstract: The trend toward parallel multi-service operation in wireless mobile communication systems has led to increasingly fragmented spectrum resources and an exponential rise in the number of RF channels, making multi-band RF circuit techniques a critical challenge. This paper presents the research progress of our group in broadband multi-band theories and methodologies, alongside the developmental roadmap for corresponding RF devices. First, the theoretical framework for multi-band impedance matching is reviewed, summarizing matching techniques for three distinct scenarios: dual-band real impedance, constant complex impedance, and frequency-dependent complex impedance. Building upon this foundation, the design and implementation of multi-band passive components, including filters, power dividers, couplers, and antennas are introduced. Furthermore, the design of multi-band active amplifier circuits and their simplified linearization methods are discussed. Currently, dual-band circuit design and fabrication encompass a comprehensive range of active and passive circuits, featuring frequency separations exceeding two to three octaves and a cumulative dual-band bandwidth of up to 1.1 GHz. Significant progress has also been made in tri-band circuit design and realization, and a variety of representative tri-band active and passive circuits have been implemented. Finally, the paper discusses future development trends in multi-band RF technologies.