Research and development in the field of gasoline engines today have to face a double challenge: on the one hand, fuel consumption has to be reduced, while on the other hand, ever more stringent emission standards have to be ful-filled. A very promising technology for simultaneous achievement of both ultra-low emissions and a high fuel economy is HCCI (Homogeneous Charge Compression Ignition) combustion. For the present study, a single-cylinder SI research engine has been modified for homogeneous compression ignition combustion by applying special cam profiles and variable exhaust cam phasing. The engine has been run both in port injection and in direct injection mode at varying operating points with different air-fuel ratios and residual gas contents. In addition to the thermodynamic analysis of the combustion process at various injection timings, the residual gas content has been examined using a gas sampling valve. Besides, optical investigations have been carried out using an endoscopic multi-fibre visualization system with a high temporal resolution for cycle-resolved analysis of the ignition and combustion process. For the numerical investigations, the engine has been modeled in one- and three-dimensional CFD codes. The gas exchange process has been analyzed and optimized by one-dimensional calculations. Three-dimensional CFD calculations of in-cylinder flow and mixing processes have been carried out with the aim of analyzing the distribution of fresh charge and residual gas and the overall homogeneity of the cylinder charge obtained as a result of the gas exchange process. The combined approach applied in this study gives new insight to the nature of combustion by homogeneous compression ignition.