Our knowledge of the biochemistry and biophysics of dinitrogen fixa­ tion has developed rapidly in the 15 years since the first N2-fixing enzyme system was successfully extracted from a bacterium. This peri­ od has produced a literature that now describes the N2 fixation reac­ tion and the nitrogenase enzyme itself in sophisticated terms, though a detailed reaction mechanism at the chemical level has not yet emerged. It is the purpose of the present monograph to present an in-depth re­ view, analysis, and integration of this research as is possible with a non-contributed publication and to relate this work to considera­ tions of N2 fixation that reach beyond the confines of the biochem­ ist's laboratory. The first section is directed as much toward the general science read­ er as toward the specialist. It covers the agricultural origins of man's interest in N2 fixation and also pertinent areas of taxonomy, physiology, and evolution. Ecological aspects of the subject include a comprehensive evaluation of the nitrogen cycle leading to a sub­ stantially greater estimate of the rate of global N2 fixation than previous ones. The treatment is of a survey fashion, in part to pro­ vide a general over-view of N2 fixation and in part to provide context for the biochemistry and biophysics that follow in the second section.