The piston is the only moving part in the combustion chamber of an internal combustion engine. With the ever increasing demand for higher power output, the control of piston temperatures has become one of the determining factors in a modern successful engine design. Because of the highly dynamic environment created by the reciprocating piston, experimental testing of the cooling process is expensive and difficult. In this research project, the first steps are made towards a CFD model, that can be used to analyse the piston cooling process at DAF Trucks NV. This thesis concerns the numerical investigation of the oil jet by using LES and the RANS analysis of the impingement heat transfer. A more realistic situation is analysed by combining these results. The results show that both the jet behaviour and the resulting heat transfer can be improved by varying the operating conditions of the piston cooling jets.