This article reviews the application and technology advancement of gallium (Ga)-based liquid metal (LM) in high-frequency circuits and antennas. It discusses the material properties of common LMs, the fluidic channels used to contain LM and their manufacturing techniques, and the actuation techniques, which are all critical for the design and implementation of LM-based devices. LM’s fluidic and pliable nature, together with its excellent electrical, thermal, and rheological (i.e., fluid flow) properties, provides some unique and innovative solutions to flexible/wearable electronics, reconfigurable circuits, and antennas. This article provides a comprehensive review of a wide range of LM-enabled high-frequency circuits and antennas, including interconnects and transitions, reconfigurable passive circuits (such as resonators, filters, and couplers), switches, phase shifters, reconfigurable antennas, flexible and wearable antennas, and metamaterials (i.e., periodic materials with properties not found in nature). This article presents various design concepts and implementation techniques, highlights key capabilities, and discusses the challenges and opportunities with the use of Ga-based LM materials.