This thesis is a study of the native and naturalized species of Salix in Utah, and is based upon the following methods. A review of all available literature was accomplished in order to compile a list of species. Type specimens were obtained from the University of California at Berkeley, the Smithsonian Institution, and the Gray Herbarium of Harvard University, University of Utah, and Utah State University. About five-hundred specimens were collected in twenty counties during the fall fo 1965 and the spring of 1966. The identification of the willows in this study was based upon type specimens, annotations by Carleton R. Ball, and descriptions and keys in the literature. As a result of this study twenty-one species are known to occur in the state. Three separate keys have been compiled for the identification of species based upon vegetative characteristics, pistillate flowers, and staminate flowers. Descriptions of the species were made from the specimens that were available for study. Species distribution maps were compiled and one representative herbarium specimen is cited for each county. Habitat information obtained from herbarium labels and field observations is included. The work has been illustrated by including photographs of specimens.