The Low emission zone is one of the policies used in urban areas to improve air quality by restricting the entry into a zone to the most polluting vehicles. The first aim of this paper is to review several studies led in Europe to assess LEZ impacts on emissions, concentrations etc. The review enabled the identification of the main positive approaches, and the gaps that led to unexpected results or that showed the LEZ was less effective than expected. Then, with a view of improving the overall frame of assessment of actions, we propose the decomposition of the LEZ on mechanisms (restriction to the most polluting vehicles, car fleet turnover, the traffic shifting, modal shifting etc.) and impacts. The review about the LEZ and the mechanisms give useful insights to improve the methodology for assessing LEZ impacts.