Humans have doubled the annual rate of nitrogen fixation (transformation of inert atmospheric molecular nitrogen, N2, to reactive forms of nitrogen, Nr) in the past 150 years. Reactive N is now accumulating in the environment on all local and regional scales. Nr accumulation contributes to many contemporary environmental problems, including eutrophication, ozone formation and acidification. The dossier “Global integrated nitrogen” summarizes PBL contributions to European and global studies related to the use and distribution of reactive nitrogen. This work involves the Integrated Model to Assess the Global Environment (IMAGE).