The spread of beneficial mutations through populations is at the core of evolutionary change. A long-standing hindrance to understanding mutational sweeps was that beneficial mutations have been slow to be identified, even in commonly studied experimental populations. The lack of information on what constitutes a beneficial mutation has led to many uncertainties about the frequency, fitness benefit and fixation of beneficial mutations. A more complete picture is currently emerging for a limited set of identified mutations in bacterial populations. In turn, this will allow quantitation of several features of mutational sweeps. Most importantly, the 'benefit' of beneficial mutations can now be explained in terms of physiological function and how variations in the environment change the selectability of mutations. Here, the sweep of rpoS mutations in Escherichia coli, in both experimental and natural populations, is described in detail. These studies reveal the subtleties of physiology and regulation that strongly influence the benefit of a mutation and explain differences in sweeps between strains and between various environments.