The scavenging process is an integral part of any two-stroke internal combustion engine cycle whether it is spark ignited or compression ignited. The scavenging process is responsible for transporting the burned gases from the previous working stroke out of the combustion chamber to allow for the fresh charge or fresh air to enter for the next combustion/working stroke. This implies that the scavenging process is responsible for setting the initial condition for the combustion process, consequently affecting fuel economy, power output and emission of hazardous gases. Two-stroke diesel engines for marine propulsion are usually uniflow scavenged cross-head engines. In uniflow scavenged engines the scavenge air enters the cylinder via ports located near the bottom dead center and exits through an exhaust valve located in the cylinder head. The in cylinder flow is therefore concentrated in one direction which gives the method its name.In this study a CFD analysis of the scavenging process in the 4T50ME-X test engine at MAN Diesel & Turbo is presented. The CFD model uses the full engine geometry including a moving piston and valve combined with time resolved measurement data as boundary and initial conditions. The results are then validated with experimental data from the engine. Optical velocity measurements have been performed through a specially designed engine cover with optical access.