The use of EGR for NOX reduction is today a standard technology for diesel engines. The mixing of air and EGR is an important issue, especially for high-pressure EGR-systems. An uneven distribution of EGR between the cylinders can lead to higher overall engine emissions when some cylinders produce more soot, others more NOX than they would with a perfectly even distribution.It is therefore important to understand the processes that control the mixing between air and EGR. The mixing is influenced by both the geometry of the mixing area and the pulsating nature of the flow.The aim of this work is to point out the high importance of the pulses present in the EGR-flow. By simulation in 1-D and 3-D as well as by a fast measurement method, it is shown that the EGR is transported in the air flow in packets. This implies that the timing between intake valve opening and the positioning of the EGR packets has a high influence of the distribution of EGR between the cylinders.The ability of 1-D and 3-D simulation to predict the behavior is evaluated. It is shown how standard 1-D simulations fail to predict the pulsation effects. Furthermore, it is shown how 1-D models can be modified to give results reasonably close to the 3-D simulation results.