Chloride ingress leading to corrosion of embedded steel is the most detrimental process affecting the durability of reinforced concrete structures. Various authors have suggested methods of calculating the time to depassivation of reinforcing steel by modelling chloride penetration on the basis of a solution of Fick's second law of diffusion. This involves several assumptions, including boundary and initial conditions that may be fulfilled in idealised laboratory tests but not in real structures. The situation calls for an explicit recognition of the limited knowledge that exists regarding both detailed mechanisms of chloride ingress and threshold chloride concentrations that cause depassivation of steel in different types of concrete subjected to different conditions of exposure. These uncertainties can result in major inaccuracies when models are used to predict the long-term performance of structures. Despite the admitted lack of detailed understanding of the process of chloride penetration, tentative proposals are outlined for alternative methods of determining performance indices useful in some circumstances for specifying the durability of structures exposed to chloride salts. They depend on measurement of one parameter (steady-state diffusion coefficient or electrical resistivity) to account for the transport process and a separate parameter to account for chloride binding. The values obtained provide a means of ranking different types of concrete in terms of their expected performance when exposed to chloride environments in which diffusion is likely to be the dominant transport mechanism.