The results of the scaling theory of localization are briefly summarized. Special emphasis is placed on features which are of importance for electrical transport in amorphous semiconductors. The critical behaviour of the average d.c. conductivity near the mobility edge is used to explain the activated behaviour of the temperature-dependent conductivity. If the system is universal near the mobility edge, the prefactor of the activated conductivity is a number, which is given by the critical conductance divided by a characteristic length that may be interpreted as a mean phase breaking distance. The new and most important result is that the statistical properties of the conductance have to be taken into account when calculating the prefactor to obtain a quantitatively reasonable description. Thus mesoscopic fluctuations seem to have much wider consequences than assumed hitherto.