This report presents the results of a rapid top-level evidence review, undertaken in July and August 2020, of the grey and academic literature on flight frequency patterns globally. We have targeted the top thirty aviation markets, in terms of absolute aviation CO2 emissions. For these countries, we have provided quantitative evidence, where available, of the share of flights taken by frequent flyers. From our analysis, we have provided regional estimates that give some indication of how far air travel is concentrated amongst particular groups. Meanwhile, recent estimations by Gossling & Humpe (2020) suggest that, globally, prior to Covid-19), those who fly most often (comprising, at most, 1% of the world's population), accounted for more than half of the passenger emissions generated by air travel2.