This review examines the impacts of reactive nitrogen on the environment, human health and economies from local to global scales. About 40% of the human population depends upon food production made possible by synthetic nitrogen fertilizers. Combustion of fossil fuels adds more reactive nitrogen to air, water and soil. This distortion of the global nitrogen cycle, while raising agricultural yields, causes degradation of water and air quality, biodiversity, ecosystem services and human health. Meanwhile, reactive nitrogen deficiencies on farmland in many developing countries continue to create economic and health hardships, and accelerate land degradation. This review is intended to assist all stakeholders in understanding and assessing these challenges, and sets out the sources and impacts of reactive nitrogen and current trends in use and emissions. Case studies show the need to integrate scientific understanding and strengthen existing policies addressing the various impacts of reactive nitrogen. Recommendations are made on the assessment, monitoring, information sharing and collaboration required at different geographical scales, and across disciplines and jurisdictions, to develop and implement coherent and effective policies to address nitrogen excess and deficiency.