Filters are one of the fundamental microwave components used to prevent the transmission or emission of signals with unwanted frequency components. In general, they can be considered as an interconnection of resonator structures brought together to accomplish a desired frequency response. Up to GHz frequencies, these resonator structures are usually constructed using lumped elements such as discrete capacitors and inductors. At microwave frequencies, discrete components lose their normal charcteristics and resonators can be realized using distributed structures like quarter- or half-wavelength transmission line stubs. However, filters built using this approach are generally big, especially when high frequency selectivity is desired. Recently, sub-wavelength structures, namely split-ring resonator (SRR) and complementary split-ring resonator (CSRR), have attracted the attention of many researchers. Interesting properties of the periodic arrangements of these structures have led to the realization of left-handed materials. Furthermore, high-Q characteristics of these structures enabled the design of highly frequencyselective devices in compact dimensions. In this thesis, these two resonator structures are investigated in detail. A deep exploration of their resonance mechanisms and transmission properties is provided along with a brief survey of related literature. However, the main focus of the thesis is the design of band-reject filters based on these resonator structures. Experimental results based on measuring the scattering paramaters of fabricated prototypes are supported with computer simulations. Band-reject filters based on SRR and CSRR are compared and discussed. It is observed that both filter types have some advantages and disadvantages which make them suitable for different applications. Finally, an electronically switchable split-ring resonator structure based on PIN diodes is presented. It is demonstrated that by employing microwave PIN diodes across the slits of an SRR, the magnetic response of a SRR particle can be eliminated. This leads to the design of filters whose rejection bands can be removed electronically