A growing body of geochemical and biological evidence indicates that nitrogen-fixing (diazotrophic) plankton make a major contribution to the nutrient demand of oceanic primary producers. Regional variation in the isotopic composition of organic matter and the elemental stoichiometry of inorganic nutrients in the water column both provide insight into spatial variation in diazotrophic activity, but the distribution, identity, and activity of the organisms responsible for fixing nitrogen all remain poorly constrained and undersampled. Recent and ongoing field studies have documented the broad distribution of a variety of diazotrophs, including the colonial cyanobacterium, Trichodesmium, cyanobacteria symbiotic with diatoms, and smaller, free-living cyanobacteria. Typical rates of oceanic nitrogen fixation appear to be on the order of tens to hundreds of micromoles m^-2 d^-1, but rates as high as 3 to 4 millimoles N m^-2 d^-1 have been documented for both Trichodesmium and small, presumably unicellular, diazotrophs. Different diazotrophs seem to segregate with depth as well as regionally. For example, Trichodesmium and diatom blooms tend to aggregate at the surface while small unicellular diazotrophs appear to be distributed throughout the mixed layer. Recent field efforts have begun to resolve basin-scale variations in the relative activity of the major diazotroph groups as well as finer-scale shifts in diazotroph community composition. We will discuss these recent findings in the context of the overall contribution of nitrogen fixation to new production in the oligotrophic ocean.