This paper provides an overview of transistors for RF applications. It reviews the evolution, current status, and application of these devices. The most important developments and major milestones leading to modern high-performance RF transistors are described, as are the heterostructures that comprise the key elements of the most advanced RF transistors. Finally, an overview of different types of transistors and their figures of merit is provided, and different applications using RF transistors discussed, with special emphasis on wireless communication systems and future trends.