Air quality monitoring of PM10 and associated health studies have focused interest on the size and the number of particles emitted to, and found in, the atmosphere. Automotive sources are one of the important elements in this, and CONCAWE have completed a study of heavy duty diesel particle emissions, complementing their previously reported light duty work. This heavy duty programme, presented here, investigated the nature of particulate emissions from two heavy duty engines (representative of different emissions levels), operating on three marketed fuels, over their respective European legislative heavy duty test cycles.The programme has investigated some of the complexities associated with obtaining credible data (e.g. dilution ratios, system stabilisation time etc.). The number distributions, which were measured over a wide size range (3 to 1000 nm), have been split into two size ranges, representative of nucleation mode and accumulation mode particles. These two ranges have been analysed in terms of the effects of engine technology, fuels and test conditions.Accumulation mode particles were found to be measurable to a satisfactory level of repeatability, and were relatively insensitive to changes in test conditions. However, nucleation mode particles, which have the greatest influence on total number measurements, were found to be very sensitive to changes in sampling conditions. Consequently, conclusions about engine technology and fuel effects based on particle number measurements are critically dependent on the sampling conditions.