The transmitter power amplifier is the most significant component influencing energy consumption and thermal management considerations to reduce operating costs and improve reliability. This article presents the fundamental principles for high- efficiency Doherty power amplifiers, (DPAs), envelope elimination and restoration (EER) power amplifiers, envelope tracking (ET) power amplifiers and outphasing power amplifiers (OPAs) design approaches. A variety of real system prototypes for HF, VHF and UHF applications are evaluated for these design approaches for efficiency, RF bandwidth, modulation bandwidth and, linearity performance, and for overall complexity. Performance is also compared to conventional Class AB amplifier designs and to the increasingly popular load modulation balanced amplifier (LMBA) architecture.