The legume genus Acacia has some 1350 species and is distributed throughout the world, particularly in Africa, Asia and Australia. The genus is exploited in natural habitats and plantations for many purposes. It forms a symbiotic association with strains of at least six genera of root-nodule bacteria (rhizobia) that are also widely distributed. Many of the associations fix nitrogen from the atmosphere,but there is great variation in nitrogen-fixing specificity in both hosts and bacteria — some acacias fix nitrogen with only a small number of rhizobial strains, others are more promiscuous. Records indicate that acacias fix less nitrogen than other leguminous trees. However, this impression appears to be an artefact of the ecosystems where the measurements were made. Most assessments of acacia nitrogen fixation have been undertaken in forests or woodlands where nitrate in the soils often inhibits nitrogen fixation, whereas the nitrogen fixed by other tree legumes has usually been measured in anthropogenic ecosystems such as plantations, hedgerows and coppices, where soil nitrate is less inhibitory. We conclude that acacias have the capacity to fix useful quantities of nitrogen but that, unlike the plant itself, its symbiosis is under-utilised. Factors that might limit nitrogen fixation are considered, with the conclusion that, as with other legumes, nitrogen-fixing ability is best expressed in the absence of limiting factors, especially deficiencies of nutrients and soil moisture. There is usually a diversity of strains of rhizobia in soils where acacias grow naturally. Many of these strains do not nodulate Acacia spp. at all and many others that do form nodules have little or no capacity to fix nitrogen. However, it appears that, within the total population of naturally occurring rhizobia, there are invariably present at least some strains that are capable of fixing significant amounts of nitrogen in association with acacias. There is no convincing evidence that, in natural environments, non-infective, ineffective or poorly effective rhizobia themselves are ever a constraint on acacia nitrogen fixation. We conclude that little can be done to enhance acacia nitrogen fixation in forests or established plantations except, where economically feasible, to correct nutrient imbalances and to control pests. On the other hand, there appears to be great and inexpensive scope to use inoculation with effective strains of rhizobia to improve the vigour and nitrogen fixation of seedlings grown in plant nurseries as tube stock destined for outplanting into the field. Where outplantings are made, inoculated, well-nodulated seedlings survive better and grow faster than their uninoculated counterparts. Inoculation with rhizobial strains cultured in a peat carrier, using a procedure termed soil enrichment, is postulated as an efficient means of producing vigorous, well-nodulated, nitrogen-fixing acacia seedlings in nurseries. Implications for commercial manufacture of acacia inoculants are discussed. Three factors are especially relevant to the timeliness and significance of this review: (i) the already substantial, and expanding, scale of acacia plantings in plantations and on farms, (ii) the potential of diverse Acacia species for the reclamation of degraded landscapes, and (iii) the expanded pool of research results relating to acacias generally that has accumulated over the past 15 years. In conclusion, there are compelling arguments that acacia nitrogen fixation can be far better exploited than it has been in the past. Th is will involve effective rhizobial inoculation of seedling stock in nurseries and development of methods of inoculation of acacia seed intended for surface seeding. There seems no doubt that, properly exploited, the symbiosis has the capacity to contribute to the productivity of acacia and companion species in plantations, to the rehabilitation of eroded and salinised lands, and to the augmentation of reserves of nitrogen in the soil. Even in circumstances where inoculation is not practicable, the cultivation of acacias has the potential to enhance soil fertility and soil structure.