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This paper reviews the uses of the nitrogen-fixing trees Prosopis tamarugo (tamarugo) and P. chilensis (algarrobo) in the deserts of northern Chile where the rainfall is less than 1 mm yr−1. These forests consist of 3200 ha of natural stands and 22,980 ha of plantations. The primary use of these trees has been to support goat and sheep production through grazing of the leaves and pods. The water for these trees comes from an underground water table whose level does not fall and that appears to be fed from an aquifer whose origin must be of higher elevation in the Andes. The water table generally ranges from 3 to 10 m in depth and the underground salinity is about 1500 mg 1−1 of total dissolved solids. The height growth of the trees is significantly greater where the water table is less than 4 m below the surface. The fruit and foliage of small, medium, and large tamarugo trees and medium and large algarrobo trees were collected to determine forage yield per tree and to develop regression equations relating total pod production per tree and forage collections from smaller litter collection boxes. The mean algarrobo production (largest trees) was 31 kg per tree in 1981 and 117 kg per tree in 1982. The mean tamarugo forage production was 87 kg per tree in 1981 and 104 kg per tree in 1982. Great variation in production per tree and production between years was observed and this greatly complicates management of the trees for forage production. The digestible protein of the leaves and fruits of both species was too low to support growth and reproduction of the animals without supplementation. Management of 5000 sheep of the breeds Precoz Merino, Australian Merino, and Karakul showed that Karakul sheep were best adapted to these desert conditions. Hereford, Galloway, and Santa Gertrudis cattle were also examined. Angora goats were examined for use in the Pampa for mohair production but their long hair became a problem with the thorny trees and this breed was eliminated in favor of the meat breeds Criollo and Pampinos. A supplement feed consisting of 25% alfalfa hay, 45% wheat bran, and 30% sheep feces was used for cattle. Feces were not used in the supplement for sheep and goats. An excellent reproduction rate was obtained (90% of the cattle, sheep and goats produced an average of 1 offspring per animal bred) when a management system was used that included vitamin A,D, and E supplementation, pregnancy determination, and supplemental feed for pregnant animals.