The biggest durability problem with structures comprised of reinforced concrete is corrosion of the steel reinforcement. The most likely cause of this corrosion, which is also the most difficult to remedy, is the presence of chloride in the concrete. This chloride effectively catalyses the steel corrosion process. Because of the huge economic significance of this problem over the last 50 years there has been almost continuous research in many countries to define and categorise the problem. A lot of the research has been replicated in different countries and by particular research bodies. One of the biggest themes of this research has been to provide a chloride concentration in steel at which a significant level of corrosion occurs. This defined level is a cornerstone of most civil engineering codes and this chapter looks at why this is a gross oversimplification of the actual situation. Most literature considers the movement of ions through concrete to occur in the same manner as in an aqueous solution by concentration diffusion. In this chapter the evidence that electromigration is an important mover of certain cation and anion species is developed and also the evidence that advection can be an extremely significant transport mechanism.18