The main objectives of this thesis are to improve the energy efficiency and physical form-factor of high-power amplifiers in base station applications. As such, the focus of this dissertation is placed on the outphasing amplifier concept, which can offer high-efficiency, good linearity and excellent opportunities for system integration. With this mind set, various outphasing concepts have been studied at different levels of abstraction, starting from their fundamental operation, to very detailed efficiency and bandwidth considerations. In order to enable the future package integration of complete RF high-power amplifiers and transmitter lineups, dedicated design techniques for very compact and high-power magnetic components have been developed utilizing low-cost wire bonding techniques. Using these techniques, a very low-loss, high-current RF transformer concept was introduced that allows straight forward power scaling in RF amplifiers. Next, to demonstrate the practical use of these low-loss magnetic components with their related design flow, two very compact high-power class-E branch amplifiers were realized and tested. Expanding on these initial works, one of the key demonstrators of this thesis was a "70W fully packaged-integrated GaN outphasing amplifier", which represents a remarkable combination of high-efficiency, high-output power with a very small form factor. Finally, to facilitate the future integration of advanced switchmode outphasing systems in a single package, the design of high-voltage CMOS drivers was discussed and demonstrated. These devices can provide the essential physical link between the final PA stages with the intelligence of the forgoing TX chain, which is typically implemented in standard low-voltage CMOS technologies.