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Diesel fuel injection pumps are lubricated primarily by the fuel itself. Traditionally, fuel viscosity was used as a rough indicator of a fuel's ability to provide wear protection, but since the advent of low sulphur diesel, even some fuels of higher viscosity have been found capable of producing wear. This paper provides further insights into the main contributors to diesel fuel lubricity, their source and the impact of refinery processing. The most effective way to monitor lubricity is also considered. We have found that diesel lubricity is largely provided by trace levels of naturally occurring polar compounds which form a protective layer on the metal surface. Typical sulphur compounds do not confer this wear protection themselves rather it is the nitrogen and oxygen containing hetero-compounds that are most important. A complex mixture of polar compounds is found in diesel and some are more active than others.