In this paper, the state-of-the-art of high-frequency transistors and their prospects for application in the high-frequency parts of systems-on-chip is reviewed. It is shown that Si-based high-frequency transistors, in particular Si MOSFETs, offer very competitive performance and fulfill most of the requirements of current system-on-chip solutions. Although today III-V HEMTs and HBTs show the best performance in terms of cutoff frequency and maximum frequency of oscillation, their widespread application in systems-on-chip is rather unlikely in the medium term. The prospects of other high-frequency device concepts, such as III-V MOSFETs and carbon-based transistors, are also discussed.