Transferred‐electron devices are the only microwave devices whose operation is solely based on the bulk‐material properties of semiconductors such as GaAs or InP. These devices are mainly employed in low‐noise medium‐power oscillators up to high millimeter‐wave frequencies. James B. Gunn was the first scientist to observe the underlying physical phenomenon in an experiment and, therefore, these devices are often referred to as Gunn devices. This chapter describes the principles of operation, basic fabrication technologies, and common oscillator circuits for them. Different device structures for performance optimization, optimization procedures, and simulation tools are discussed. The chapter also summarizes the current state of the art of Gunn devices and the outlook covers the prospects of more recent device structures and material systems.