The fall armyworm, Spodoptera frugiperda (J. E. Smith) is a significant economic pest in the western hemisphere, causing substantial losses in corn, sorghum, forage, and turf grasses. Although fall armyworm does not survive severe winters, it infests most of the central and eastern United States and Canada because of annual migrations from overwintering sites in Florida and Texas. A detailed description of these movements is a prerequisite for identifying the factors that determine the timing and direction of migration and for developing models that can predict the severity of infestations at the migratory destinations. Complicating this effort is genetic heterogeneity within the species, which increases phenotypic variability. Particularly important are 2 “host strains”, defined by a preferential association with either large grasses (designated corn-strain), such as corn and sorghum, or smaller grasses (designated rice-strain), such as rice and bermudagrass. This paper reviews recent studies examining the genetic complexity of fall armyworm populations, including characteristics of the 2 strains and the possibility of subgroups within strains. The use of this information to monitor short and long distance movements is discussed.