|During the spring months, hickories throughout the Eastern U.S. become adorned with abnormal growths or galls on the leaf or stem tissue. Many of these galls are caused by phylloxerans (Hemiptera: Phylloxeridae: Phylloxera spp. Boyer de Fonscolombe) which are small, cryptic, phytophagous insects that are closely related to aphids. Their galls come in an array of shapes, sizes, and colors and are considered extended phenotypes of their inducer (fundatrix). Hickory-feeding phylloxerans mainly cause aesthetic damage to the trees they infect, but some species, such as Phylloxera russellae Stoetzel, that infect pecans can cause economic damage. Despite their ubiquity and possible economic importance, hickory-feeding phylloxerans have been the focus of few taxonomic studies and have not been studied extensively for over 100 years. There are only 32 described hickory-feeding species in the U.S., and most of the taxonomy is based on collections from a small area around Washington, D.C. In addition to a lack of knowledge about their diversity in the U.S., even less is known about their evolutionary relationships. The three objectives of this dissertation are to: 1) Perform the first estimate of the phylogenetic relationships among phylloxeran species and test the monophyly of accepted genera and hosts; 2) Revise the species diversity; and 3) Characterize their gall morphologies and hosts. To accomplish these objectives, Phylloxera galls and/or phylloxerans were collected from across the U.S. on a variety of hosts including one species of Castanea Mill., 11 species of Carya Nutt., three species of Quercus L., and two species of Juglans L. DNA sequence information of mitochondrial COI and nuclear Ef-1α was used to estimate the phylogeny. Phylloxera was determined to be monophyletic and Phylloxera spp. cluster by gall morphology rather than host. Fourteen putative new hickory and walnut-feeding Phylloxera spp. were described, and several new host records (black hickory, nutmeg hickory, sand hickory, and Northern California walnut) were discovered. With the additions made here and three species being designated as nomina dubia (Phylloxera foveata (Shimer), Phylloxera globosa (Shimer), and Phylloxera minima (Shimer)), the total number of Phylloxera spp. feeding on hickories and walnuts in the U.S. is now 44. A key is provided to gall forming phylloxerans in the U.S.