This dissertation is a study of the phylogeny and evolutionary biology of gall formation in psyllids of the subfamily Spondyliaspidinae, with particular focus on North American hackberry gallers in the genus Pachypsylla. Species in this genus produce a variety of gall types on the leaves, petioles, buds and twigs of their hosts, four species of Celtis subgen. Euceltis (Ulmaceae). The homogeneity of adult morphology in Pachypsylla, contrasted to the great variation in gall morphology and phenology, has led to much difficulty in delimiting species. Chapter I investigates species limits as related to gall type and host specificity in Pachypsylla. Strong differences in allozymes, morphology and life history confirm that leaf, petiole, bud and twig gallers belong to different species or species groups. Different leaf gall morphs probably also represent different species, as evidenced by significant allozyme frequency differences among sympatric pairs of gall morphs, consistent frequency difference between co-occurring morphs across localities, and discrete differences in gall type between progenies of individual females. Differences in allozymes, female phenology, adult and nymphal coloration, as well as laboratory rearings and field manipulations, show that side cell individuals within two nipple gall types represent an inquiline sibling species (Chapter II). Chapter III is an analysis of phylogenetic relationships within Pachypsylla, based on allozyme, morphological, life history and chromosome characters. Galler populations attacking the same plant tissue form monophyletic groups. The leaf galler morphs are little diverged, and phylogenetic relationships among them are unclear. Populations of inquilines from two different gall types appear closely related; the inquiline appears to be derived from a gall-forming ancestor. Phylogenetic relationships among gallers on different plant parts are consistent with an evolutionary sequence of gall position from leaf to petiole to bud to twig. Chapter IV is a morphological study of phylogenetic relationships within Spondyliaspidinae. The tribe Pachypsyllini, including Pachypsylla and two related Celtis feeders, is monophyletic. The tree favors the hypothesis of Burckhardt over that of White and Hodkinson. The distribution of lerp and gall formation is shown to be non-random within Spondyliaspidinae.