The book covers the various approaches to modeling the in-cylinder processes such as mixture formation, combustion and formation of exhaust emissions in diesel and gasoline engines. Due to their complexity, emphasis is put on multi-dimensional spray, combustion and emission formation models. However, phenomenological as well as zero-dimensional thermodynamic models, which are still widely used in engine development because of their computational efficiency, are addressed as well. Example calculations of each model type are compared with corresponding experimental data -- represented in diagrams as well as in images resulting from modern optical measuring techniques -- in order to discuss the capabilities of today's simulation models and the shortcomings that still exist either because of oversimplifying assumptions or insufficient knowledge. Readers achieve an overview of the most important simulation models describing the in-cylinder processes of internal combustion engines and gain insights into which modeling approach is appropriate for a specific problem.