Novel quadruple-mode, dual-mode, and dual-band filters as well multiplexers are presented. A cylindrical dielectric resonator sized appropriately in terms of its diameter D and length L will operate as a quadruple-mode resonator, offering significant size reduction for dielectric resonator filter applications. This is achieved by having two mode pairs of the structure resonate at the same frequency. Single-cavity, quad-mode filters and higher order 4n-pole filters are realizable using this quad-mode cylindrical resonator. The structure of the quad-mode cylinder can be simplified by cutting lengthwise along its central axis to produce a half-cut cylinder suitable for operation in either a dual-mode or a dual-band. Dual-mode, 2n-pole filters are realizable using this half-cut cylinder. Dual-band filters and diplexers are further realizable using the half-cut structure and full cylinder by carrying separate frequency bands on different resonant modes of the structure. These diplexers greatly reduce size and mass of many-channel multiplexers at the system level, as each two channels are overloaded in one physical branch. Full control of center frequencies of resonances, and input and inter-resonator couplings are achievable, allowing realization of microwave filters with different bandwidth, frequency, and Return Loss specifications, as well as advanced filtering functions with prescribed transmission zeros. Spurious performance of the half-cut cylinder can also be improved by cutting one or more through-way slots between opposite surfaces. Size and mass reduction achieved by using the full and half-cut resonators described, provide various levels of size reduction in microwave systems, both filter level, and multiplexer level.