Measurement procedures for the legal limitation of emissions from heavy duty engines should adequately reflect the actual use of such engines in practice. Equally important is the possibility to monitor the emision performance of certified engines on the market. This paper gives a short overview of the experiences with the Dutch in-use compliance program for HD vehicles. The results indicate that such a monitoring of the existing fleet is of great importance for the national authorities. This means that any measurement procedure envisaged for legislation purposes should take account of the requirement of relatively easy monitoring of the emission behaviour in service just as much as the requirement of adequate simulation of practical vehicle operation.