The permeability of concrete to liquids, ions and gases is of direct relevance to both durable concrete and to leak-resistant concrete for containment. Analytical models used to predict the age at which corrosion of the reinforcement will be initiated require a detailed knowledge of the transport mechanisms involved and the permeability of the concrete. Some specifications for projects in extreme environments (e.g. the Middle East) or requiring very long lives (e.g. railway tunnels) have specified permeability criteria, based on some form of analytical durability model, to be verified by testing the supplied concrete or precast concrete elements. However, there does not appear to be any agreement on the test methods that should be adopted. This Report, which is an updated version of TR 31 published in 1988, reviews how permeability can be measured and what typical results are achieved. In preparing the Report, the Working Party has paid particular attention to in situ permeability, either with site cast or precast concrete.