"Micro electro mechanical systems (MEMS) applications in radio frequency (RF) and microwave electronics are revolutionizing the progress in wireless communications. In particular, MEMS switchable/tunable capacitors can facilitate the realization of multistandard software defined transceivers, offering better performance than existing solutions. In this thesis, characterisation and RF modelling of MEMS switchable/tunable capacitors and the technical requirements for their application in multiband wireless transceiver circuits in a software defined radio (SDR) are described. The first device considered is a curled cantilever capacitive switch, configured in both shunt and series configuration where the capacitance is limited to two stable states. The second device, referred to as a Hill shaped Zip-up Capacitor (FIZC), is a novel fixed beam structure where capacitance is continuously variable between the top beam and the central electrode. This thesis has developed a new and simple method for constructing 3D geometries of non planar MEMS structures in different actuation states for accurate electromagnetic modelling. This method combines mechanical and RF model tools and excellent model accuracy has been demonstrated for both MEMS switched and tunable capacitors. This model can be used to study the effect of surface roughness and can predict the air gap value in the lower contact electrode of MEMS capacitive structures. The RF performances of the capacitors were simulated by integrating the structures with coplanar waveguides with design improvements which extend the Isolation, capacitor tuning and Quality factor. In addition, the circuit with an equivalent lumped elements model is simulated and the CLR (capacitance, inductance and resistance) values are extracted. The applications of HZC capacitors in a reconfigurable impedance matching network are designed and analyzed. A simple lumped element impedance matching network was simulated at a single frequency with HZC tunable capacitors to match the impedance of power amplifier and the antenna and the results are presented."