Book excerpt: The book systematically introduces the design theory and method of multi-band RF filtering circuits for the modern wireless communication systems or radar systems, which are required to operate at multi-bands. These multi-band filtering RF circuits have drawn more and more attention from the engineers and scientists in the field of RF circuits design. The book proposes the detailed theoretical analysis and abundant experimental data of multi-mode resonators, multi-band bandpass filter with high selectivity, reflectionless multi-band bandpass filter, balanced filter with high suppression, slotline based multi-band balun filter, switchable filtering diplexer based on reused L-shape resonator and miniaturized 55-/95-GHz on-chip dual-band bandpass Filter. The book is intended for undergraduate and graduate students who are interested in filtering circuits design, researchers who are investigating RF & microwave systems, as well as design engineers who are working in the RF & microwave circuits field. Readers can get an in-depth understanding about the multi-band RF filtering circuits design theory and method.