About 3500 species represent the Psylloidea across the world. Many Psylloidea live on a wide range of agriculturally and horticulturally important plants and some of them also act as vectors of plant pathogens. Generally they show a arrow host-plant range and feed on plant sap. Endosymbiotic bacteria are shown to be associated with some of them, enabling them to live on a nutritionally imbalanced plant diet. During feeding, the Psylloidea induce changes in plant tissues. Salivary enzymes such as pectinases enable them to mobilize primary metabolites rapidly to feeding sites from uninfested parts. Specific proteins (64 and 58 kDa) occur in the saliva of free-living Psylloidea (e.g. Aphalaridae) as well as in host-plant phloem. These insects live either freely or by constructing lerps or by inducing galls. Variations in guilds and feeding behaviour determine the nutritional ecology and physiology of the Psylloidea. Varying nutrient levels in leaves regulate populations of the gregariously feeding Psylloidea. The lerp-constructing Psylloidea utilize more of sugar-based nutrients, while the groupfeeding Psylloidea induce more intense changes in amino-acid, fatty-acid, and mineral levels in host plants. High C and low N ratios in leaves influence psylloid growth rates negatively. For instance, the gall-inducing Psylloidea achieve only two generations a year. High levels of the sterol (440.3 molecular weight) and ergosterol and low levels of complex lipids in young leaves of E. macrorhyncha appear to regulate the specificity of the gall-inducing species of Glycaspis (Synglycaspis) (Aphalaridae). About 100 plants are indicated as hosts of Indian Psylloidea. Curiously no lerp-forming psylloid is known in India.