Symbiotic biological nitrogen fixation in terrestrial ecosystems offers critical ecosystem functions, alleviating nitrogen limitation, stimulating ecosystem recovery and promoting carbon sequestration. When the growth of most plant species is limited by soil nitrogen availability, the highly diverse group of nitrogen-fixing plant species can form a symbiosis with mutualistic bacteria that break the triple bond of atmospheric dinitrogen, giving fixers access to the large atmospheric pool of nitrogen. However, despite this advantage, nitrogen-fixing plants exist at a range of abundances and dominate in very few ecosystems. This observation suggests that the trait of fixation must, in some environments, reduce the fitness and competitive ability of fixers. Given the importance of symbiotic nitrogen fixation, the environmental changes facing many ecosystems and the complex world in which the trait persists, it is therefore critical to determine the factors that govern the evolution, abundance and function of fixation in terrestrial ecosystems. Several mechanisms have been invoked to govern symbiotic nitrogen fixation. These include constraints by soil nutrients, water or herbivory and the costs of deterring cheating bacteria, providing resources to symbionts or maintaining the genetic architecture needed for nodulation and fixation. Counterbalancing these forces, plants have evolved many strategies to minimize these costs and constraints and to maximize the benefits of fixation. Multiple mechanisms may act at once, reflecting the complexity in which nitrogen-fixing species have evolved. To understand why fixation has not evolved to become more prevalent across terrestrial ecosystems, an examination of this complexity must bridge a range of scales, from genes to cells to physiology to individual plants to communities and to whole ecosystems. Therefore, we need a cross-disciplinary approach, incorporating techniques and perspectives that draw from genetics, targeted greenhouse and field-based experiments and theoretical modelling. This session will bring together some of the leaders studying symbiotic nitrogen fixation that represent a range of disciplines, backgrounds and scales and will seek to synthesize some of the recent advances in the study of the ecology and evolution of symbiotic nitrogen fixation.