Distribution functions for the entrainment and transport of individual sediment particles have been derived by numerous workers in the literature. Although these functions completely describe the erosion process, they are often considered less than satisfactory for modelling because of their stochastic nature. This paper demonstrates the linkage of sediment-transport theory to the development of such transport functions to show that this approach is the only realistic one available for most contexts. Furthermore, it is able to overcome a number of important limitations when upscaling to the plot or hillslope scale. The derivation of techniques for such upscaling and their practical application is discussed.