The amount of emitted pollutants from an internal combustion engine is regulated by emission legislation. Commonly regulated pollutants for the diesel engine are NOx and PM. Exhaust gas recirculation (EGR) is one efficient way of controlling the NOx emissions, and to control PM emissions an accurate lambda control is used.Both EGR- and lambda control requires good knowledge of the gas flows in the engine. The gas flows of interest are inlet air, EGR, total gas flow through the engine and total amount of exhaust gas. There are several possible concepts to measure and/or model these gas flows, all with their pros and cons.Flow and concentration based measurement concepts for determining the gas flows in a heavy duty diesel engine with EGR are investigated. The flow based concepts measures the amount of gas directly with a flow meter such as a hot-film air meter, ultrasonic flow meter or an orifice plate. Concentration concepts measure the concentration of a gas component (i.e. oxygen). In both concepts the measurement is used together with fuel flow, a combustion model and a volumetric flow model to determine the gas flows. Both flow and concentration concepts are evaluated in simulations. The simulation output is then compared to the experimental results.It is shown both in the theoretical and the experimental study, that for EGR and lambda control, flow based concepts are superior to concentration based concepts. The conclusion is also that high amounts of EGR can be calculated with sufficient accuracy from a measured air flow and an estimated gas flow through the engine and the only way to accurately determine really small amounts of EGR is by measurement.