An account is given of the four types of microwave generator made from semiconductors: the transistor, the varactor, the avalanche diode and the transferred electron diode. The design problems for each device are outlined. A more detailed discussion of the physical principles of avalanche diodes and transferred electron diodes is included. The two types of transferred electron device, the Gunn diode and the LSA diode, are contrasted. Finally, comparisons are made between the performances of all the generators, including power output and noise, and some indication is given of future prospects.