The Malacothamnus alliance consists of three genera, Iliamna, Malacothamnus, and Phymosia. The genera are considered taxonomically complex since hybridization freely occurs, polyploidy levels vary, and there is a lack of distinct morphological characters to delineate taxa. Several taxonomic treatments have been prepared for each genus, but relationships within the genera and the relationship of the Malacothamnus alliance to others in the Malvaceae remains unknown. This multifaceted study aimed to (a) examine the monophyly of the Malacothamnus alliance and its position in the Malvaceae, (b) determine the relationships between genera in the alliance, (c) compare variation of nuclear and chloroplast genes in the alliance, (d) prepare revised taxonomic treatments for Iliamna and Malacothamnus, and (e) examine the probability of successful hybridization in Iliamna. The monophyly of the Malacothamnus alliance was not confirmed using DNA sequences of both nuclear and chloroplast regions. In Iliamna, little sequence variation was detected among taxa in the Rocky Mountains; however, the nuclear and chloroplast regions conflicted with regard to the relationships of the western and eastern taxa. An ancestral copy of the chloroplast genome is shared between the two eastern U.S. Iliamna species and Phymosia (Bahamas and Mexico). The nuclear ITS sequences indicated the western U.S. Iliamna species were more closely related to Phymosia and Malacothamnus than to other species in Iliamna. Neither data set provided sufficient variation to resolve relationships of species in Malacothamnus. Genetic variation and the feasibility of hybridization in Iliamna supported the results of the broader phylogenetic studies. Iliamna corei and I. remota are recently derived from I. rivularis. Hybrid offspring of I. corei and I. remota had higher viability and fecundity than did hybrids between crosses of either species and I. rivularis. The Virginia populations of I. corei and I. remota are more genetically similar than either is to Illinois populations of I. remota. However, the species are morphologically distinct and can easily be distinguished from others in the genus. Revised taxonomic treatments for Iliamna and Malacothamnus based on surveys of herbarium material are presented. Taxonomic revisions include the new combinations of Iliamna grandiflora subsp. grandiflora and I. grandiflora supsp. crandallii and the resurrection of Malacothamnus hallii and M. orbiculatus. / Ph. D.