Floral nectar,a sweet, aqueous secretion that consists mainly of sugars and to a lesser extent amino acids, is an effective reward that entomophilous plants could provide for their pollinators. Plant-pollinator relationships mediated by nectar characteristics have been recently studied in pollination biology. Floral nectar is initially sterile, but some yeasts and bacterial species survive in floral nectar when they have been dispersed into nectar via air or by flower-visiting insects such as pollinators. The identification of microorganisms in floral nectar from broad-scale surveys has shown that nectar yeasts are widespread and occur in a wide range of habitats. No such surveys are currently available for bacteria, but limited studies suggest that bacteria may also be widespread in nectar. However, high sugar content and consequentially low water activity that characterize the nectar of many plant species limit the number of microbial species that are capable of surviving and proliferating in the harsh environment of nectar, leading to species-poor communities with one or a few culturable species per nectar sample. Other factors affecting nectar-dwelling microorganism communities are nitrogen availability, pH, sugar composition, temperature and the presence of secondary or antimicrobial compounds. The presence of yeasts and bacteria in nectar decreased sugar concentrations and altered sugar ratios. Besides, the metabolic activity of nectar-dwelling microorganisms affected other floral attractive traits, including temperature, scent and amino acid content. In some cases, changes in physical and chemical properties of nectar affected on pollinator behavior and as a result on plant reproductive success. Therefore, nectar-dwelling microorganisms and their ecological functions are of great interests among pollination biologists. In the future, molecular and chemical analysis technologies should be combined to further reveal the mechanisms of potential factors that might affect the shaping of nectar-dwelling microorganism community, to fully elucidate the possible mechanisms that nectar-dwelling microorganisms alter nectar physico-chemical characteristics and the plant-pollinator relationships, and to further understand the ecological roles that nectar-dwelling microorganisms might play in different ecosystems.