Biological nitrogen fixation (BNF) is the distinguishing feature of a legume in a cropping system.Most legume species are able to form a symbiosis with alpha- or beta-proteobacteria, collectively called rhizobia, that use solar energy captured by the plant to break the bond in inert atmospheric dinitrogen and form reactive N species, initially ammonium (NH4+).As a result of this symbiosis, the legume crop requires little or no input of N fertiliser and makes little demand on soil N reserves.Large, continental- or global-scale estimates of BNF have been attempted but there are many uncertainties in data. We set out to develop an improved estimate for BNF across Europe.BNF is usually quoted on a kg ha-1 basis from experiments conducted in a limited area and time, and datasets from a wide range of crops and growing conditions show a wide range in amounts fixed, depending especially on biomass and available soil N.Previous attempts to estimate N fixation have tended to follow a common approach in that they calculate the amount of N fixed as the as the product of the land area cultivated and fixation per area.The figures for the former are obtained from published databases such as FAOstat and, subject to the level of detail made available about data collection methods, are taken at face value.In contrast, data on N fixation range hugely for each legume species depending on growing conditions, in particular biomass production and available soil nitrogen.Yields range more than 10-fold across species, countries and years.Thus we took the approach that it is not desirable simply to use legume areas for each country and convert these to overall fixed N using an average figure for BNF.Hence we attempted to take a stepwise approach using the most detailed available data and robust scientific logic.