Filtering devices, including planar and spatial filters, are some of the most basic components in RF front-end systems to select the desirable signals in operating frequencies and prevent interferences from signals of other bands. Generally, filters are cascaded with other devices, such as power dividers, antennas, couplers, or other massive modules, which could introduce unexpected interference, discontinuity and more losses. Therefore, multifunctional filtering devices, such as filtering antennas, filtering power dividers, frequency selective surfaces, etc., are further researched. The mode-inspired concept is attractive for the realization of advanced performance for these filters. Diverse filtering techniques based on mode-inspired method need to be extensively explored for the development of microwave and millimeter-wave components to apply in 5G+ and 6G communication systems.