Xylophagous leafhopppers are common and abundant insects of tropical and subtropical environments and play important ecological roles in these ecosystems. The feeding biology of these insects is unique in terms of their high feeding rates and a digestive physiology that allows them to assimilate amino acids, organic acids, and sugars at approximately 99% efficiency. For those species well studied, fluctuations in plant xylem chemistry and tension appear to determine the diurnal and seasonal use of their host plants. Relatively few species of xylem fluid-feeding leafhoppers are considered important pests in commercial agriculture, as they transmit the bacterial plant pathogen Xylella fastidiosa. X.fastidiosa induces diseases of grapevines, citrus, coffee, almond, alfalfa, stone fruits, landscape ornamentals, and native hardwoods for which there is no cure. Two Xylella diseases, citrus variegated chlorosis (CVC) and Pierce's disease (PD) of grapevines, have emerged as important issues within the past decade. In Brazil, CVC became important in the early 1990s and has now expanded throughout many citrus-growing areas of South America and threatens to spread to North America. The recent establishment of the exotic glassy-winged sharpshooter (Homalodisca coagulata) in California now threatens much of the United States' wine grape, table grape, and almond production. The spread of H. coagulata throughout southern California and the spread of CVC northward from Argentina through Brazil exemplifies the biological risks from exotic species. The occurrence and epidemiology of leafhopper-vectored Xylella diseases are discussed.