Publisher Summary This chapter presents the concepts of mediated transport. The movement of many types of substances across cell membranes could be accounted for by the process of their dissolution in the lipid phase of the membrane and their subsequent diffusion through this lipid phase to leave the membrane at the opposite face. Yet, there were many substances that could not be included within such a simple model. For these substances, permeation was much faster than the simple model would have predicted, permeation was often not a linear function of substrate concentration, and was often inhibitable by analogues of the substrate or by reagents capable of attacking proteins. For all such deviant substances, one might suggest that there exist membrane components, probably proteins, which mediate the movement across the membrane of these substances. Such mediated transport may be a net movement of substrate down its electrochemical gradient, in which case, the process is often termed facilitated diffusion. In other cases, the net movement of substrate may be against the prevailing electrochemical gradient, when the phenomenon is termed active transport. The chapter discusses in turn, both these forms of mediated transport, emphasizing the very strong links between them. The approach is to start with the very simplest model for mediation, to see what its predictions are, and then to introduce successively more complex models.