Biological nitrogen fixation is a key process in ecosystems, but its ecological controls are not well understood. This review examines the factors that influence nitrogen fixation in free-living cyanobacteria, vascular plant symbioses, and heterotrophic bacteria. The study highlights the importance of trace elements, grazing, and phosphorus limitation in controlling nitrogen fixation. A model of cyanobacterial nitrogen fixation in lakes suggests that trace-element limitation and zooplankton grazing can constrain cyanobacteria in estuaries, maintaining nitrogen limitation. Similarly, a model of symbiotic nitrogen fixation on land suggests that shade intolerance, phosphorus limitation, and grazing on nitrogen-rich plant tissues can suppress nitrogen fixers in late-successional forest ecosystems. These findings raise the question of why many tropical forests contain nitrogen-fixing legumes, while nitrogen fixers are absent from most temperate and boreal forests. The study suggests that high nitrogen availability in tropical forests allows legumes to maintain a nitrogen-demanding lifestyle without needing to fix nitrogen. Overall, both simulation and conceptual models of ecological controls of biological nitrogen fixation suggest that there are common features across nitrogen-fixing organisms and ecosystems. Despite the diversity of organisms and ecosystems involved, these common controls provide a foundation for developing regional and global models of nitrogen fixation. The nitrogen cycle poses many challenges, including the question of why nitrogen limitation is so important to many terrestrial and aquatic ecosystems. The availability of nitrogen is a limiting factor for productivity, composition, dynamics, and diversity in many ecosystems. Despite the abundance of nitrogen in the atmosphere, nitrogen fixation is limited by factors such as trace elements, grazing, and phosphorus limitation. The study emphasizes the need to understand these ecological controls to develop better models of nitrogen fixation in ecosystems.Biological nitrogen fixation is a key process in ecosystems, but its ecological controls are not well understood. This review examines the factors that influence nitrogen fixation in free-living cyanobacteria, vascular plant symbioses, and heterotrophic bacteria. The study highlights the importance of trace elements, grazing, and phosphorus limitation in controlling nitrogen fixation. A model of cyanobacterial nitrogen fixation in lakes suggests that trace-element limitation and zooplankton grazing can constrain cyanobacteria in estuaries, maintaining nitrogen limitation. Similarly, a model of symbiotic nitrogen fixation on land suggests that shade intolerance, phosphorus limitation, and grazing on nitrogen-rich plant tissues can suppress nitrogen fixers in late-successional forest ecosystems. These findings raise the question of why many tropical forests contain nitrogen-fixing legumes, while nitrogen fixers are absent from most temperate and boreal forests. The study suggests that high nitrogen availability in tropical forests allows legumes to maintain a nitrogen-demanding lifestyle without needing to fix nitrogen. Overall, both simulation and conceptual models of ecological controls of biological nitrogen fixation suggest that there are common features across nitrogen-fixing organisms and ecosystems. Despite the diversity of organisms and ecosystems involved, these common controls provide a foundation for developing regional and global models of nitrogen fixation. The nitrogen cycle poses many challenges, including the question of why nitrogen limitation is so important to many terrestrial and aquatic ecosystems. The availability of nitrogen is a limiting factor for productivity, composition, dynamics, and diversity in many ecosystems. Despite the abundance of nitrogen in the atmosphere, nitrogen fixation is limited by factors such as trace elements, grazing, and phosphorus limitation. The study emphasizes the need to understand these ecological controls to develop better models of nitrogen fixation in ecosystems.