A simple theory is outlined, based on the conservation of energy, whereby such diverse processes as wind-blown and river-transported sediments, mud-flows and sand avalanches can be brought together and explained in terms of the established concepts of general physics. It is shown that the mean rate of transport of sediment in turbulent suspension is determined by the whole size distribution of the transported sediment rather than by any arbitrarily defined mean size. This suggests that more detailed studies of the changes which occur in size distributions during the cycle of erosion transport and deposition would throw much-needed light on the underlying processes.