The modelling of internal and external flow phenomena of high pressure injection systems has developed significantly in the last few decades. The challenge currently however, is to model these areas of flow together accounting for different multi-phase phenomena that require a wide range of resolutions. More specifically, the highly turbulent nature of internal nozzle cavitation requires a high grid and temporal resolution, whereas the external nozzle atomisation processes exists over a comparatively much larger space with lower time and space resolution needed. The research described in this thesis is focused on coupling the internal nozzle and external processes using a single Eulerian Volume Of Fluid (VOF) multiphase model. First, investigations into the dynamics of internal nozzle cavitation is presented, through simulation of two phase nozzle flows without spray formation demonstrating sensitivities to discretization techniques and boundary conditions. Then, the simulation of internal nozzle cavitation with spray formation using a single model was achieved by the construction of a three phase VOF model with cavitation which is described. A non-condensable gaseous phase is considered alongside the liquid and vapours phases, the liquid interface is sharp with a diffusive interface between gaseous phases. Comparisons were made with both experimental data and previous numerical investigations. Finally, a new solver with the introduction of the Eulerian-Lagrangian Spray Atomization (ELSA) framework with the Interface Capturing Method (ICM) for surface density to the system to describe the liquid structures below the Sub-Grid Scale (SGS) is presented. Thus quantities such as droplet Sauter Mean Diameter (SMD) and droplet spray angle at these scales can be extracted, with comparisons made with experimental data. The coupling of the ELSA-ICM model with the three phase cavitating model allows for processes of the entire spray formation to be resolved. More specifically, for the first time, the evolving surfaces of the entire injection process, from internal nozzle cavitation to spray atomisation, can thus be tracked throughout even at the SGS. This allows for a direct insight into the interaction between the cavitation and atomisation processes.