This dissertation mainly focuses on the developments of balanced bandpass filters and their applications. In the beginning, novel symmetric four-port balanced bandpass filters are proposed to exhibit the desired differential-mode bandpass response and also suppress the common-mode signal. Specifically, by properly designing the symmetric resonators and making good use of virtual-open/-short boundaries at the plane of symmetry, it is possible to give the desired differential-/common-mode frequency responses. To demonstrate the design concept, three most widely used filter structures, lumped-element structure, coupled-line structure, and coupled-resonator structure are utilized in developing the four-port balanced bandpass filters. Next, based on the transformation conditions for converting a symmetric four-port network into a three-port balun, novel three-port balun lumped filters and balun coupled-resonator filters are implemented from the developed symmetric four-port balanced filters. Owing to the possibility of minimizing the common-mode noise, the resultant in-band amplitude-/phase-balance performances can further be optimized. Finally, benefited by the well-designed balun bandpass filters, three applications with significant improvements are presented. First, a compact LTCC (low-temperature co-fired ceramic) bandpass 180° hybrid is implemented to integrate bandpass filter and 180° hybrid. Second, a balun diplexer that combines necessary passive components into a single device is presented to reduce circuit size and improve system performance. At last, a wideband quasi-Yagi antenna fed by the proposed wideband balun bandpass filter is realized to maintain stable co-polarized radiation patterns, low cross-polarized radiation levels, and a well-defined return-loss response throughout the wide passband.