Sugar feeding is a fundamental characteristic of mosquito life. Most evidence indicates frequent ingestion by both sexes and all ages of mosquitoes of plant sugar, usually as floral and extrafloral nectar and honeydew. Energetically, sugar and blood are interchangeable; females of some species have evolved independence from one or the other, but most need blood to develop eggs and sugar to survive, to fly, and to enhance reproduction. Mosquitoes' commitment to sugar is further illustrated by a wealth of behavioral, structural, and physiological specializations for finding, feeding on, and processing it. Blood and sugar feeding activities are antagonistic and mutually exclusive, owing to conflicting demands, yet they support the same goals and often share the same activity period. The rules by which females make food-choice decisions have been inadequately explored, and we still lack convincing evidence that sugar availability in nature varies sufficiently to affect mosquito populations.