Diesel particulate filter (DPF) is the most effective emission control device for reducing particle emissions (both mass, PM, and number, PN) from diesel engines, however many studies reported elevated emissions of nanoparticles (<50 nm) during its regeneration. In this paper the results of an extensive literature survey is presented. During DPF active regeneration, most of the literature studies showed an increase in the number of the emitted nanoparticles of about 2-3 orders of magnitude compared to the normal operating conditions. Many factors could influence their amount, size distribution, chemical-physical nature (volatiles, semi-volatiles, solid) and the duration of the regenerative event: i.e. DPF load and thermodynamic conditions, lube and fuel sulfur content, engine operative conditions, PN sampling and measurement methodologies. Moreover some experimental tests were performed at Istituto Motori’ labs with three Diesel vehicles (a Euro 5 2.2 liter van, and two medium size passenger cars) to estimate the effect of regeneration events in terms of both occurrences and emissions output. ELPI (Electrical Low Pressure Impactor) by Dekati, sampling directly from the tailpipe with a double stage dilution FPS, was employed to measure the size distributions of the total emitted particles in the range 7 nanometer up to 10 micron. Regeneration events were studied during NEDC, WLTC and Artemis driving cycles and they exhibited considerable variations in the time for cleaning as well as in the amount of PN emissions.