The present research describes an investigation of the flow through the inlet port and the cylinder of an internal combustion engine. The principal aim of the work is to interpret the effects of the port shape and valve lift on the engine's "breathing" characteristics, and to develop a better understanding of flow and turbulence behaviour through the use of Computational Fluid Dynamics (CFD), using a commercial available package STAR-CD. A complex computational mesh model was constructed, which presents the actual inlet port/cylinder assembly, including a curved port, a cylinder, moving valve and piston. Predictions have been carried out for both steady and transient flows. For steady flow, the influence of valve lift and port shape on discharge coefficient and the in-cylinder flow pattern has been examined. Surface static pressures predicted using the CFD code, providing a useful indicator of flow separation within the port/cylinder assembly, are presented and compared with experimental data. Details of velocity fields obtained by laser Doppler anemometry in a companion study at King's College London, using a steady flow bench test with a liquid working fluid for refractive index matching, compared favourably with the predicted data. For transient flow, the flow pattern changes and the turbulence field evolutions due to valve and piston movement are presented, and indicate the possible source of cyclic variability in an internal combustion engine.