Biological N(2) fixation is an important part of the marine nitrogen cycle as it provides a source of new nitrogen that can support biological carbon export and sequestration. Research in the past decade has focused on determining the patterns of distribution and abundance of diazotrophs, defining the environmental features leading to these patterns and characterizing the factors that constrain marine N(2) fixation overall. In this Review, we describe how variations in the deposition of iron from dust to different ocean basins affects the limiting nutrient for N(2) fixation and the distribution of different diazotrophic species. However, many questions remain about marine N(2) fixation, including the role of temperature, fixed nitrogen species, CO(2) and physical forcing in controlling N(2) fixation, as well as the potential for heterotrophic N(2) fixation.