Volumes 1 and 2 of the Manual of Nearctic Diptera were concerned with the anatomy, terminology, and identification of all the known families and genera of two-winged flies from America north of Mexico. This third volume, dedicated to the late Willi Hennig (1913-1976), explains the rationale underlying the classification that was adopted in Volumes 1 and 2, using the methodology of phylogenetic systematics developed by Hennig (1950, 1965, 1966) and based largely on his most recent phylogenetic treatment of the order (Hennig 1973). The subject matter of Volume 3 was originally intended to appear as a single chapter on the phylogeny of Diptera, to be written by Hennig himself. However, his manuscript was only in draft at the time of his death and the editors were reluctant to make the required changes and additions without his permission. Since then, many new facts and ideas have accumulated, which have necessitated an entire volumes devoted to this subject. A few of these ideas do not entirely correspond to the most recent opinions on Hennig, and some of them are not even in complete accordance with the classification that we, ourselves, utilized in Volumes 1 and 2. We make no apologies for these changes because classification is not static but is subject to revisions that result from new information or interpretation.