The characteristics of typical 1/fnoise spectra which have been observed, and the physical models proposed for their explanation, are reviewed. In m.o.s. transistors the 1/fspectrum extends over at least ten decades, and the effect of the gate field on the frequency index has been investigated. It is argued that these measurements support the theory that the conductivity of the inversion layer fluctuates due to electrons tunnelling to and from traps in the silicon oxide dielectric. It is shown that the tunnelling model satisfies Halford's statistical criterion.