Although biological control of the gypsy moth, Lymantria dispar (L.), has been studied in the northeastern United States for the past 70 years, and with greatly renewed interest in recent years, no comprehensive identification keys to the parasites of the gypsy moth have been available. This paper supplies keys to adults and puparia of the flies of the family Tachinidae that are parasites of the gypsy moth. Included are (1) all important exotic species and all species that have been introduced, whether or not presently known to be established; (2) important parasites of the closely related and dispar-like Lymantria obfuscata Walker from India, some of which have already been introduced; and (3) North American species that have been reared from the gypsy moth, even though they are only “minor” parasites, at least at the present time. The keys, supplemented by illustrations and by a glossary of technical terms, have been designed for use by professionals or laymen. Moderately detailed descriptions of 34 species are followed by concise summaries of the distribution, taxonomy, immature stages, biology, and hosts of each species, and its history and status as a parasite of the gypsy moth, with special reference to North America. One new species, originally from northern India, is named and described. In a final section, brief notes are given on 47 species that are minor and incidental parasites of the gypsy moth but whose names may be encountered in the literature.