The lecture 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 presented. Heterostructures which are key elements of the most advanced RF transistors, are described, and an overview of the different transistor types and their figures of merit is given. Applications of RF transistors with special emphasis on wireless communication systems and future trends are addressed.