The greenhouse gas emission reduction targets agreed at Kyoto represent a first step to reduce emissions in the long term in order to stabilize the earth's climate. The European Union has made an important contribution to the Kyoto agreement and the European Commission intends to develop a strategy to reach the Union's Kyoto target. This will require action in all sectors of the economy including the transport sector. Of the six gases covered by the Kyoto protocol, carbon dioxide (CO\d2) is the most important as it accounts for about 80% of the total global warming potential of all six greenhouse gases. In the European Union, the share of transport CO\d2 emissions in total increased from 19% in 1985 to 26% in 1995. Road traffic is the most important source, and largely determines the trend in the transport sector; and road freight accounts for about 35% of transport CO\d2 emissions. As well as CO\d2, road freight transport causes considerable amounts of other pollutant emissions. With a share of approximately 75% of particulate (PM) emissions and about 60% of oxides of nitrogen (NO\dx) emissions, road freight transport is the most important source of these pollutants within the total transport sector. To forecast and monitor the development of heavy-duty vehicle (HDV) emissions and the effect of emission-reducing measures, calculation methods must be provided. COST 346 deals with the subject of estimation of air pollutant emissions caused by heavy- duty vehicles. International research should be structured to today's needs and a European database of emission-related information for heavy-duty vehicles should be developed. Basic data for emission estimates, emission models and activity data will be included. COST 346 will cover the results from national research activities as well as those from the CEC framework research programs ARTEMIS and PARTICULATES.