The more and more stringent regulations on particle emissions at the vehicle tailpipe have led the car manufacturers to adopt suitable emissions control systems, like particulate filters with average filtration efficiency that can exceed 99%, including particulate mass (PM) and number (PN). However, there are still some specific operating conditions that could exhibit noticeable particle number emissions. This paper aims to identify and characterize these persistent sources of PN emissions, based on tests carried out both at the engine test bench and at the chassis dynamometer, and both for Diesel and Gasoline direct injection engines and vehicles. For Diesel engines, highest particle numbers were observed downstream of the catalyzed DPF during some operation conditions like engine warm up or filter regeneration phases. PN could be 50 times higher during the warm up phase and can reach as much as 2000 to 3000 times more during the regeneration phase compared to normal operation. For GDI engines, tests have shown high concentrations of ultrafine particles at the tailpipe, with a factor that can be 15 to 300 times higher than those observed at the tailpipe of modern Diesel engines whatever the operating conditions, though highest PN emissions were especially detected during cold operating phases and harsh accelerations. These particles are mainly solid, since the semi-volatile particles are easily converted by a standard 3 way catalyst. Additional evaluation in simulated electrically hybridized operation has also shown that strong PN emissions could occur during the restart phase of the thermal engine.