A review of experimentally and commercially available therapeutic systems that use radio-frequency (RF) and microwave energy is provided in the first part of this article. The second part considers new advanced electrosurgical systems that combine the advantages associated with the use of low-frequency RF energy and high-frequency microwave energy to enhance the overall clinical effect and discusses how the frequency of operation and the design of the antenna structure can be optimized to ensure the desired tissue effects are achieved. Consideration is also given to how the latest developments in high-frequency semiconductor power technology developed for the communications sector are enabling new microwave and millimeter-wave energy-based electrosurgical systems to be developed and commercialized at an affordable cost.