The discovery of symbiotic nitrogen fixation, announced a century ago in Berlin, and published in full two years later by Hellriegel & Wilfarth, ended 60 years of controversy and ushered in the modern era of intensive investigation, the extent of which was not then foreseen and which continues unabated. Hellriegel & Wilfarth's great contribution was in proving that legume nodules fixed atmospheric nitrogen. They also showed that the nodule-inducing `ferment' was to some degree specific, that it occurred in different abundancies in different soils, that it was killed by moderate heat and harmed by drought, that the nitrogen fixed by a nodulated legume was not immediately available to plants growing alongside and that small quantities of combined nitrogen did not affect nodulation, whereas larger amounts were inhibitory. This remarkable achievement does not detract from the historic work done much earlier by Boussingault, Lawes, Gilbert and others, who showed by meticulous experiment that neither legumes nor other plants could fix atmospheric nitrogen, in spite of the capacity of the former to enrich themselves, and the soil in which they grew, with combined nitrogen. Earlier workers were so concerned with the need to exclude from their experiments any chemical contamination with ammonia, dust, etc., that they also excluded microorganisms and thus failed to discover fixation. Hellriegel & Wilfarth worked with the light infertile soils of North Germany, where addition of nitrogen was essential. This probably contributed to their success; differences in growth between legumes with and without nodules were quickly seen. Advances in nitrogen fixation research over the past century encompassed many areas in chemistry, biology and agriculture. That nitrogen fixation is among the key processes sustaining life was established by this work, but many fundamental aspects remain obscure, especially in the genetics and physiology of the functioning symbiosis. The catalogue of nitrogen-fixing organisms and associations, and their detailed description, is still incomplete; some nitrogen-fixing systems are in urgent need of conservation. A deeper understanding of these matters will improve our ability to manage the cycling of nitrogen in agricultural and other ecosystems so as to increase protein yields of crops and avoid environmental and energy problems associated with intensive methods of production based wholly on fertilizer nitrogen.