Aphid-killing flies of the genus Sphaerophoria Lepeletier & Serville (Syrphidae) of the Western Hemisphere are revised. Thirteen species, including 5 new species, are recognized: novaeangliae Johnson (Novaeangliae group); contigua Macquart, pyrrhina Bigot, sulphuripes (Thomson) (Contigua Group); scripta (L.), philanthus (Meigen), abbreviata Zetterstedt, brevipilosa, n. sp., longipilosa, n. sp., cranbrookensis Curran, asymmetrica, n. sp., bifurcata, n. sp., and weemsi, n. sp. (Scripta Group). Study of type-specimens revealed the identity of 3 unrecognized species, but 3 other names are placed here as nomina dubia (cleoac Metcalf, interrupta Jones, and nigricauda Metcalf). Sphaerophoria infumata Thomson, dubia Bigot, pachypyga Bigot, and melanosa Williston are placed as synonyms of sulphuripes; nigratarsi Fluke is a synonym of philanthus; guttulata Hull is a synonym of pyrrhina; and fulvicauda Bigot is a synonym of contigua. A distribution map is presented for each species. Published information on biological aspects, including all prey records for Western Hemisphere species, and on the morphology of the immature stages, is reviewed.