This report presents an overview of how humans have altered the global nitrogen cycle. An introduction to the nitrogen cycle is provided; as well as human induced nitrogen fixation and its impact on the atmosphere, carbon cycle, aquatic and terrestrial ecosystem function, and biodiversity. The importance of nitrogen as a primary building block of life and its role in biological processes is also explained. Ecosystems that evolved under nitrogen limitation or under other constraints are now inadequately adapted to it. This results in adverse effects such as eutrophication which itself leads to decreased biodiversity Recommendations and management implications in reaction to future human activities that further disrupt the nitrogen cycle are discussed. Issues in Ecology is an ongoing series of reports designed to present major ecological issues in an easy-to-read manner. This Issue summarizes the consensus of a panel of scientific experts based on the information that was current and available at the time of its publication in 1997.