The standing crop of nectar is the primary attractant to pollinators in many species of flowering plants in north temperate habitats. However, basic biological data on nectar and its secretion are needed. Although nectar tissue structure has been examined in a few species, nectar constituents including sugar and non-sugar fractions, nectar secretion rates and timing, and standing crop, in most species are unknown. Yet many features of production and specific constituents of nectars influence the prefernce of foragers. In this paper, I elucidate some of the factors of nectar biology important in analyses of pollinator attraction to plants and ultimately in pollination success of the plants.