Cicadellinae includes approximately 340 genera and over 2,000 species distributed worldwide, with member commonly referred to as sharpshooters. Sharpshooters are specialists on xylem sap, one of the nutritionally poorest diets, along with related cicadas and spittlebugs. Statistical analyses using phylogenetic contrasts of hemipteran body sizes did not show a significant increase in xylem feeding lineages, but results should be viewed cautiously. Sharpshooters host two mutualistic bacterial endosymbionts to complement their poor diet. Phylogenetic analyses were conducted based on the 16SrDNA from Baumannia and Sulcia and on COI, COII, 16SrDNA, and H3 for 29 leafhopper hosts. A congruent evolutionary history of both Baumannia and Sulcia with their sharpshooter hosts is supported based on all (Baumannia) or most (Sulcia ) statistical tests conducted here, suggesting a long-term association of these bacteria with their hosts. Additionally in this thesis, studies on the taxonomy of economically important genus Homalodisca are presented. Finally, incongruence in the higher-level classification of Cicadellinae is mainly due to the lack of robust phylogenetic hypotheses, like the one presented in this thesis. A morphological study based on 183 characters coded for 170 taxa and a molecular study based on partial regions of COI, COII, 16S rDNA, and H3 gene sequences from 91 taxa. Results support changes in the higher-level classification of Cicadellinae including: the erection of the tribe Oncometopiini based on previous members of the Proconiini; the treatment of Phereurhinini within the subfamily Cicadellinae; and placements of 9 genera in different tribes. Furthermore, based on a combined analysis, the origin of the egg-powdering behavior and related sexually-dimorphic morphological characteristics were studied. Results suggest a single origin of the egg-powdering behavior, possibly in the ancestor of Phereurhinini and Oncometopiini. Modifications of the female hindlegs for scraping the brochosomes off onto the egg nests were also acquired once in the ancestor of the Oncometopiini, while modifications on the female forewing setation for better anchoring of brochosome pellets, seem to have been acquired multiple times. Multiple losses of the behavior and its related associated traits occurred in various oncometopiine lineages.