Almost a century ago, Hellriegel and Wilfarth demonstrated that legumes fixed N2 with the aid of microorganisms living in root nodules (Hellriegel, 1886a; Hellriegel and Wilfarth, 1888). They finally explained an observation, made by Boussingault a half-century earlier, that legumes gathered more nitrogen than other plants. Since then, microbiologists, ecologists, chemists, biochemists, agronomists, botanists, geneticists, and, more recently, molecular geneticists have investigated the problems of biological N2 fixation. Our perception of this important process has evolved from the empirical observations of ancient farmers on the beneficial effects that legumes have on the soil to the point today where many of the genes involved in N2 fixation have been isolated and their nucleotide sequences determined. The meaning of nitrogen fixation has changed from the early 19th century to the present. Originally the term implied the fixing of any nitrogen into organic matter, whether combined nitrogen or N2. Today, the term nitrogen fixation is most commonly used when referring to the conversion of N2 to the oxidation state of ammonia—a process carried out biologically only by prokaryotes.