The aim of this research is to develop a low-voltage, high-power and high-efficiency power amplifier for mobile and wireless communication applications. Class-E amplifier was selected as the core configuration because of its special features and simplicity of the circuit. The circuitry of Class-E amplifier is simple but the design process is very difficult as the transistor is operated in switching mode and there is no well-established method in determining the circuit component values. A detailed mathematical analysis is conducted and a MathCad program is developed for conventional Class-E amplifier design. The proposed design procedures and computer program are validated with a 100kHz Class-E amplifier constructed using bipolar transistors. A novel two transistors circuitry is developed in order to increase four times the output power of Class-E amplifier. In such a circuit, two transistors are operating in push-pull mode. One of them is used to handle positive input cycle and the other one acts on the negative cycle. Thus, the overall voltage covered by the amplifier is doubled and hence high power output with low voltage supply can be obtained. Good agreements between the design and measured parameters are acquired. Since the target applications of the project are in mobile and wireless communications, a high commercial value will be obtained if the high-frequency (> 900MHz) Class-E amplifier can be realized using MOS technology. This is the major objective of this research. However, the large and highly nonlinear parasitic capacitance makes the design more difficult in this approach as compared to the bipolar transistor technology. A piece-wise linear approximation was therefore developed for dealing with the junction capacitance in the MOS device. In this approximation an analytical study can be conducted and the circuit design procedures can be greatly simplified. Results show that the amplifier characteristics computed with the newly developed design formula agree very well with the PSPICE simulations. A novel Class-E amplifier structure, along with its modelling and design procedures was proposed. Also several high-frequency amplifiers were designed for practical applications. Class-E amplifier arrays for providing higher output power (with power added efficiency ranging from 60% to 73%) at operation frequencies of 0.9GHz, 1.8GHz and 2.4GHz were designed. 'Their pass band is very flat and the bandwidth is wide enough to accommodate the frequency ranges of the currently used wireless networks. In addition, the amplifier power control can be achieved with a DC-DC converter voltage control, as required in wireless portable devices.