Abstract: Over the past 40 years ambient air quality in Brussels improved significantly. This was especially true for sulfur dioxide, lead, nitrogen monoxide, carbon monoxide, benzene and Benzo a pyrene. With respect to the air quality objectives imposed by the most recent European directive on air quality, 2008/50/EC, two major problems remain: nitrogen dioxide (NO2) and particulates (PM10 and PM2.5). Although the air quality objectives are met at several measuring sites in Brussels, a thorough analysis of data shows that it will be impossible to become fully compliant, in due time, in all of the different urban environments. A comparison of the average concentration levels in the Brussels Capital Region with those in the surrounding regions, the interpretation of the average daily and weekly concentration profiles and some special observations (e.g. car free Sundays) make clear that drastic emission reductions will be needed if compliance is to be assured solely by measures on the local scale.