Progress in Diesel spray modelling highly depends on a better knowledge of the instantaneous injection velocity and of the hydraulic section at the exit of each injection hole. Additionally a better identification of the mechanisms which cause fragmentation is needed. This necessitates to begin with a precise computation of the two-phase flow which arises due to the presence of cavitation within the injectors.For that aim, a VOF type interface tracking method has been developed and improved (Segment Lagrangian VOF method) which allows to describe numerically the onset and development of cavitation within Diesel injectors. Furthermore, experiments have been performed for validation purpose, on transparent one-hole injectors for high pressure injection conditions. Two different entrance geometries (straight and rounded) and various upstream and downstream pressure levels have been considered. This numerical approach allows to retrieve different cavitation regimes and a good agreement has been obtained for the discharge coefficients. Encouraging results have also been achieved concerning the emission frequency of the cavitation pockets at the injector exit. Then preliminary calculations have been performed on a VCO Diesel injector with needle displacement and an estimation of the injection velocity has been obtained for this configuration.Finally, the VOF method has been applied to calculate directly the three phase flow (liquid and vapour Diesel fuel, and external gas) downstream the injector exit. This method should give better insight, in a near future, into the mechanisms of fragmentation.