Dinitrogen fixation is a key process in the N cycle and only carried out by few prokaryotes. Research on dinitrogen fixation includes basic and practical applications: from nif genes to crops, with molecular, genetic, ecological, taxonomic, and agricultural approaches used. Nitrogen fixing rhizobia, which have been used in agriculture for over a 100 years, are excellent research models still leading the knowledge of eukaryote-bacteria symbioses. Other less known symbioses of dinitrogen fixing bacteria are reviewed as well as free-living diazotrophs.