Concern for engine particle emission led to EC regulations of the number of solid particles emitted by LDV and HDV. However, all conventional piston-driven combustion engines emit metal oxide particles of which only little is known. The main sources are abrasion between piston ring and cylinder, abrasion of bearing, cams and valves, catalyst coatings, metal-organic lubrication oil additives, and fuel additives. While abrasion usually generates particles in the μm range, high concentrations of nanosize metal oxide particles are also observed, probably resulting from nucleation processes during combustion. In general, metal oxides, especially from transition metals, have high surface reactivity and can therefore be very toxic, especially nanosize particles, which evidently provide a high specific bioactive surface and are suspected to penetrate into the organism. Hence, these particles must be scrutinized for quantity, size distribution and composition.Published data are summarized and data from investigations of various engines with respect to metal oxide particle emission are reported. These investigations were performed without and with VERT-approved particle filters, where VERT is an international verification standard for emission reduction technologies, which, besides of filtration effectiveness, durability and limited pollutants also includes the analysis of secondary emissions, potentially formed by these technologies and of size specific metal emissions.In good agreement with literature, the overall metal mass in the exhaust of IC engines without particle filter is in the range of 0.1-1 mg/km metal. This combines wear metals and metals from lubrication oil additives. Size-specific chemical analysis has shown that a large part of metal oxide particles are to be found in the size classes below 60 nm. However there are more metal oxide particles in the exhaust attached to soot particles of larger size, as chemical analysis also revealed. If there are less soot particles prevalent, like at idle conditions some of them do appear unattached in a separate fraction of much smaller size. SMPS particle size distribution at idle shows peaks of up to 108 particles per cc in the size range of 10-30 nm. It must be assumed that these are all metal oxide particles since PMP sampling was applied which means that these particles survived 300°C and thus cannot be volatiles. This high number of solid metal oxide particles implies a potential health risk. Hence, there is a need to further focus on small metal oxide particle emissions. For diesel engines, industry has demonstrated that particle filters are available which can very efficiently filter those nanoparticles. There is little known about metal oxide emissions of other engines but it must be anticipated that all IC piston engines do emit such particles. Elimination of such metal oxide particles by highly efficient filtration therefore might become an urgent future requirement for all engine categories.