A systematic treatment of the genus Aloysia is presented. It contains 30 species and 14 varieties distributed across North and South America. It is a member of the tribe Lantaneae and is most closely related to the genus Lippia. The history, chromosome numbers, secondary chemistry, generic and infrageneric relationships, morphology, cladistics, and taxonomy of Aloysia are all addressed. A complete index to all known names and a list of exsiccatae is provided for the approximately 6120 sheets examined from 25 herbaria. Four new species are described, A. arequipensis, A. coalcomana, A. cordata, and A. velutina, and three new combinations are proposed, A. virgata var. urticoides, A. lycioides var. schulziana, and Lippia dodsoniorum.