Mexico City Metropolitan Area (MCMA), once identified as the most polluted city in the world few decades ago is now a history of success in terms of air quality. As a result of a series of air quality actions implemented over the last 25 years, air quality has improved considerable. Based on a robust air monitoring and health databases, a recent study have shown that reductions in PM2.5 in this period, avoided 22,500 premature deaths approximately. Still, there is more way to go since quality standards are not attained yet for ozone and particulate matter (PM2.5 annual average concentrations in 2017 were 23 μg/m3 and 30 μg/m3, in Mexico City and Metropolitan area monitoring sites, respectively). Existing data either from emissions inventories, continuous monitoring and field campaigns have shown the contribution of the different emissions sources, highlighting the important role of transportation in emissions and formation of ultrafine particles in MCMA. Few pilot studies have shown the high level of ultrafine particles emissions of the existing heavy duty diesel fleet (Euro II and III). In addition, results of ultrafine particles emissions of gasoline light vehicles measured at the inspection and maintenance test shown that old vehicles among other characteristics are also high emitters. ... mehrIn despite of the above, ultrafine particles emissions and/or ambient concentrations are not yet part of the policy and/or regulation discussion in Mexico. The intention of this paper is to present a review of existing information of ultrafine particles in MCMA in order to analyze the impact of existing and planed control measures for air pollutants on ultrafine emissions and formation.