Astragalus, excluding Astracantha (formerly Astragalus subgenus Tragacantha), has a world total of c. 2500 species, of which c. 500 are in the New World; it is one of the largest genera of flowering plants. The estimated number of species in the Flora Iranica area is c. 900–1000. Because of the huge size of the genus, no one person is familiar with all its groups; although 150 sections have been described, many are poorly delimited, often not typified and many untenable. In the Old World the genus is almost entirely restricted to the northern hemisphere. Sections with primitive characters mostly contain mesophytic species widespread in mountains and relatively humid regions; in the more advanced sections the species are adapted to dry or arid conditions.
Only a few sections are strictly endemic to the Flora Iranica area (e.g. sects. Microphysa, Acanthophace, Leucocercis). In 2 of the better known large sections (Astragalus and Caprini) there are 2 main centres of diversity: the Armenian knot and the Zagros mountains; Afghanistan and the adjacent Pamir-Alai region. Their patterns of distribution and migration are probably repeated in several other sections in the genus.