[en] Since the end of 2000, a continuous sampling to monitor PCDDs and PCDFs emission was implemented on the 11 municipal waste incineration ovens in Walloon Region, to check the compliance with the EU emission limit value, 0.1 TEQ ng/Nm³. For this purpose, uncertainty estimation is one of the most crucial points for decision making. The sampling and analytical uncertainty was estimated by two different ways: 1. Identification and quantification of the major contributions through QA/QC data (type A evaluation) or by other sources (type B evaluation), 2. Direct assessment through specific duplicate measurements. Both ways were found to give results in good agreement, with an extended uncertainty of about 30 to 40%, depending on the congener, with a coverage factor k=2.