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Four variants of Class E power amplifiers (PAs) were examined in this thesis, namely classic zero voltage switching (ZVS), classic zero current switching (ZCS), newly proposed ZVS, and newly proposed ZCS. Suboptimum operation of the classic ZVS Class EPA was thoroughly investigated and from these investigations some important figures of merit emerged. A key finding from this investigation is that in suboptimum mode the amplifier can operate at a higher frequency than the optimum mode would permit while a deployment of a larger device is also possible for higher power application. The effect of the transistor ON resistance on the performance of classic ZCS PA was studied and design equations derived which allow us to re-compute the optimal circuit component values in the presence of transistor ON resistance were presented. It was observed that using the optimal rather than non-optimal component values the output power degradation due to the presence oftransistor ON resistance can be increased to the desired level. A newly proposed ZCS Class EPA (Inverse Class E) was analysed and from this it was revealed that the intuitively speculative prediction (when it was first introduced) stating that this amplifier type was superior to the classic Class EPA due to its less sensitivity to the transistor ON resistance was proven to not entirely correct. However, from further investigations it emerged that this Class E variant has a number of advantages· relative to the classic Class EPA such as lower peak switch voltage, higher load resistance, and the potential for higher efficiency at increased output power. From its characteristics, the Inverse Class E topology may benefit from the present trend toward lower-power (and hence lower-breakdown voltage) switching transistor technologies. A modified Inverse .Class E PA ('proposed ZVS Class E') was then introduced in order to extend the usefulness of the original Inverse Class E (that is limited only for low-to.-medium power applications since no shunt capacitance representing transistor output capacitance is incorporated in the circuit) to high-power applications. Also it was shown that the proposed ZVS Class E when operated as a frequency multiplier with even output harmonic production is superior to the classic Class E arrangement. In this work the author constructed two classic Class E PAs operating at 2.4 GHz from a 3 V dc supply voltage, i.e., one with two-harmonic termination and the other one with three-harmonic termination in which the latter offers more than 10% higher drain efficiency and 23 dB better third-harmonic suppression level than the former. The PA employing three-harmonic termination delivered 19.2 dBm output power, achieved peak power added efficiency (PAE) of 60%, drain efficiency of 69%, and exhibited 9 dB power gain. The Inverse Class EPA has also been experimentally validated in which the constructed PA delivered 22 dBm output power at 2.3 GHz and achieved peak PAE of 64%, drain efficiency of 69%, and 11.6 dB power gain when operated from a 3 V supply voltage. The ability of these PAs to handle a constant-envelope modulated signal such as GMSK and Bluetooth GFSK has been confirmed with no spectral re-growth observed in both cases. Further experimental study of the phase noise response of the Inverse Class EPA showed the same trend as predicted by conventional phase noise theory. Dual-band Class-E PAs which through the use of the recently developed composite right/left-handed transmission line (CRLH TL) concept do not require band switching or diplexers were described in this thesis. As part of this work, new power combining techniques were developed for both classic ZVS Class E and Inverse Class E PAs as a means to boost output power and to effectively deliver this power into an unbalanced load without the need for a BALUN. When operated in a Linear Amplification using Nonlinear Components (LINe) or outphasing transmitte~, these power combining circuits offer improved instantaneous efficiency at the expense reduced output power when compared to the traditional circuit deploying a Wilkinson combiner. This thesis also presents an adaptation. to the classical I1Q modulator topology which simultaneously allows it to operate both as a vector modulator and as a high efficiency balanced amplifier. Here experimental evidence for the behaviour ofthe modulator when operated in QPSK mode at 2.33 GHz with a data rate of 1 Msymbol/s shows that EVM of less than 5% with amplifier's PAE of 65% is possible.