A brief outline is given of oscillator noise theory in order to introduce the concept of a noise measure as a means of defining a figure of merit for oscillator devices. Details are then given of a.m. and f.m. noise measurement techniques as applied to devices operating at millimetre wave frequencies. Using these techniques a comparison is made of the relative noise performance of both GaAs and InP transferred electron devices (t.e.d.s) and Si double-drift Impatt devices. It is thus shown that GaAs and InP t.e.d.s exhibit very similar noise performances with InP having the advantage of a higher power and efficiency capability. More surprisingly the results also prove that the Si double-drift impatt can exhibit low-noise behaviour under certain bias conditions. Finally it is concluded that the choice of oscillator devices for millimetre wave uses is more complex than previously thought and the double-drift impatt is now a real contender for low-noise systems use.