This paper reviews some of the progress that has occurred recently in both the theory and performance of resonant-tunneling diodes. It begins by describing the present physical understanding of the resonant-tunneling process. Recent experimental advances in resonant-tunneling oscillators are then discussed, including the demonstration of oscillations at frequencies up to 200 GHz. To better understand these results, a detailed analysis is made of the mechanisms that limit the speed of this device. This analysis includes a calculation of the quasibound state lifetime, a determination of the device capacitance from the electrostatic band-bending, and an estimate of the negative differential conductance using two different methods. On this theoretical basis, a device structure is analyzed that could oscillate up to 600 GHz.