In microwave circuits, power amplifiers (PAs) are essential components for power amplification. In this era of 4G and 5G technologies, the need for PAs is rising with wider bandwidth and higher drain efficiency to reduce DC power consumption. PAs can be categorized into three main types based on their biasing, operating mode, and resulting drain voltage/current waveforms: conventional Class PAs, switching class PAs, and harmonically tuned PAs (HTPAs). Using convectional PAs achieving 100% drain efficiency is difficult. Switching Class PAs theoretically offer 100% efficiency but are limited by transistor switching speeds at high frequencies. HTPAs utilize waveform-shaping techniques to achieve high drain efficiency and wider bandwidth, making them ideal for 4G, 5G, and 6G applications. This chapter explores the theory, principles, and design method of HTPAs, including Class F, inverse Class F, Class J, continuous Class F, and extended continuous Class F PAs.