Taxonomic interpretations of the North American genus Robinia (Leguminosae) have been diverse, the number of species recognized varying from four to twenty. Field and herbarium studies, supplemented by cytological data and phenetic analysis now show clear distinctions between four species of Robinia: R. pseudoacacia L., R. hispida L., R. viscosa Vent., and R. neomexicana Gray. The latter two species each include two varieties. R. hispida L., consisting of four varieties, is a complex that includes a wide range of morphological variability. The preponderance of populations of the Piedmont and Coastal Plain Provinces of the southeastern United States are sterile, mostly triploid clones, reproducing only by root suckering. Populations that are both fertile and diploid are limited to the mountains of western North Carolina and adjacent eastern Tennessee. Preliminary cytological investigations indicate that sterility is due to either abnormal meiosis and non-functional gametes as a consequence of triploidy, or an unknown genetic mechanism in sterile diploids, resulting in failure to set fruit. Phenetic analyses of the species, their subordinate population groups (varieties), and occasional hybrids and hybrid swarms between species, largely corroborates cytological, exomorphic, and geographic data. The phenetic presentation includes dendrographs from cluster analyses and a three-dimensional plot from principal components analysis;A generic description and a key to species is provided, and each taxon is accompanied by a description, key to varieties (if applicable), list of synonyms, distribution map, and illustration. One new combination is presented: Robinia neomexicana Gray var. rusbyi (Woot. & Standl.) Peabody.