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. The process of hydrotreating to reduce sulphur levels also destroys some of these natural lubricants. Other refinery processes also influence the concentration of the lubricity agents in the final fuel blend. Lubricity additives have been developed to compensate for the deterioration in natural lubricity observed in low S diesels. The interaction between natural polars and lubricity additive has been investigated and the findings may explain why some poor lubricity fuels are more responsive to lubricity additive than other. Difficulties are encountered when using knowledge of refinery streams to predict the lubricity of a diesel blend. The most effective way to monitor lubricity performance is by making measurements on the finished fuels. Vehicle tests have shown that the High Frequency Reciprocating Rig is a good indicator of diesel lubricity performance.