The book covers the various approaches to modelling, measuring and optimising the mixture formation process in internal combustion engines. Due to their complexity and importance in predicting the temporal and spatial distribution of liquid and gaseous fuel inside the cylinder, special emphasis is put on the detailed description of multi-dimensional CFD-models regarding the flow conditions inside the injection nozzle, the primary and secondary spray break-up, evaporation, turbulent dispersion, collision, wall impingement and ignition. Example calculations are compared with semi-empirical relations and corresponding experimental data - represented in diagrams as well as in images resulting from modern optical measurement techniques - in order to discuss the capabilities of today's simulation models as well as their shortcomings. Moreover, today's and future requirements concerning the development of injection systems capable of optimising the combustion process in terms of reduced fuel consumption and emissions are discussed. The reader will also be provided with an overview of the most important optical measurement techniques for the investigation of mixture formation in internal combustion engines.