The upper frequency of operation for a semiconductor microwave device is usually considered in terms of the equivalent circuit parameters associated with the immediate neighbourhood of the active element, together with any stray reactances of the package. Whilst it is generally recognized that in the semiconductor contribution both the resistive and reactive terms may have a frequency dependence, the exact nature of this is difficult to determine experimentally and its importance is generally obscure. This paper seeks to give a brief survey of the various mechanisms which can conceivably introduce frequency-dependent effects and attempts to gauge which of them is likely to be important in devices of present interest—crystal valves, varactors, tunnel/backward diodes and p-i-n switches. The mechanisms discussed include such items as skin effect, carrier/phonon collisions, dielectric relaxation, carrier lifetime, transit time, etc.