At part load and wide open throttle operation with stratified charge and lean mixture conditions the Direct Injection Spark Ignition (DISI) engine offers similar efficiency levels compared to compression ignition enginesThe present paper reports on results of recent studies on the impact of the in-cylinder processes in DISI engines e. g. the injection, the in-cylinder flow, the mixture preparation and the ignition on the combustion, the energy conversion and the exhaust emission behavior. The analyses of the spray behavior, of the in-cylinder flow during compression as well as of the flame propagation have been carried out applying advanced optical measurement techniques. The results enable a targeted optimization of the combustion process with respect to engine efficiency and exhaust emissions. The benefits of an increase in fuel injection pressures up to 100 MPa are discussed. The investigations have been mainly performed in a single cylinder research engine featuring a spray-guided injection process with central injector location. The considered operating range includes part load conditions with both stratified and homogeneous mixture preparation as well as full load operation with homogeneous mixture preparation. The results obtained from the research work are very promising and hence a 4-cylinder production engine is modified to operate with a suitable injection system with rail pressures in the range of 50 MPa to 60 MPa.Additionally, irregular combustion events in the full load regime such as knocking combustion and low speed pre-ignition have been investigated in a 4-cylinder DISI production engine. The results support the development of mitigation strategies for both engine knock and low speed pre-ignition (LSPI).