One of the aims of the Bioethanol for Sustainable Transport (BEST) project is to demonstrate the environmental benefits of ethanol as an alternative to fossil-based transport fuels. The BEST Evaluation Plan provides a systematic framework for monitoring and assessment of those benefits, based on a detailed set of evaluation requirements for the demonstration activities in the project. These requirements include monitoring of ethanol usage at the project sites, reviewing the available evidence for the environmental impacts of ethanol use, and where necessary, carrying out new analyses to improve our understanding of those impacts. This report is a review of the available evidence for the air quality impacts of using ethanol fuels in transport. The report focuses on regulated and unregulated air pollutant emissions from vehicles running on ethanol fuels, but does not cover greenhouse gas emissions. The review begins with a general overview of the air quality impacts of burning fuels in vehicle engines, listing the types of pollutants normally produced and their impacts on human health and the environment. The factors influencing air pollutant formation and standard methods of limiting pollutant production and emission are described. The specific impacts of using ethanol in both petrol and diesel engines are then considered, both from the perspective of what current knowledge indicates those impacts should be, and through considering the reported results of recent studies.