During a monographic treatment of the genus Baptisia, interest was aroused in the state of taxonomic confusion into which its closest generic affinity, Thermopsis, had fallen. According to the "Index Kewensis" and the Gray Herbarium Catalogue, it is comprised of thirty species: eight Asiatic, not considered in this revision; and twenty-two North Americanfour from the southeastern United States, and eighteen from the western United States. The latter group constitutes a western geographical supplement of Baptisia whose western limits roughly coincide with the eastern limits of Thermopsts. The problems were found to be largely bibliographical. The present treatment proposes no novelties, but has indicated two new combinations. It consists largely of a drastic reduction of a number of species. The four heretofore recognized southeastern species are reduced to three, and the eighteen western species have been reduced to seven species and five varieties. A key to the species, and brief descriptions with complete citations of synonyms and type or authentic specimens are given. Relative uniformity, particularly within the western group, makes delimitation of entities difficult in many cases. As in Baptisia, flower structure furnishes little of value in this respect, and has therefore been given little emphasis. Fruit and vegetative characters seem to be the best bases for the separation of species, and have been stressed accordingly. Considerable doubt is felt by the author as to the validity of the two varieties of T. rhombifolia: annulocarpa and arenosa, for they