Silicon-based optical microcavities are very popular for many applications because of the ultra-compact footprint, easy scalability, and functional versatility. In this paper we give a discussion about the challenges of the optical microcavities on silicon and also give a review of our recent work, including the following parts. First, a near-“perfect” high-order MRR optical filter with a box-like filtering response is realized by introducing bent directional couplers to have sufficient coupling between the access waveguide and the microrings. Second, an efficient thermally-tunable MRR-based optical filter with graphene transparent nano-heater is realized by introducing transparent graphene nanoheaters. Thirdly, a polarization-selective microring-based optical filter is realized to work with resonances for only one of TE and TM polarizations for the first time. Finally, a on-chip reconfigurable optical add-drop multiplexer for hybrid mode- /wavelength-division-multiplexing systems is realized for the first time by monolithically integrating a mode demultiplexer, four MRR optical switches, and a mode multiplexer.