Many species of insects exhibit wing dimorphism, one morph having fully developed wings and the other morph having reduced wings and being incapable of flight. These wing dimorphisms provide visible manifestations of migratory polymorphisms. Since winged individuals do not, in principle, have to fly, the existence of forms with reduced wings suggests that there is a tradeoff between flight capability and other fitness components. Comparisons of the life histories of the fully winged and wing reduced morphs demonstrate that this tradeoff is most commonly expressed as a decrease in the age of first reproduction and increased fecundity in the morph with reduced wings. Given these tradeoffs, the evolution of wing dimorphism will depend upon its genetic basis, including correlations with other life history components. A review of the recent literature suggests that the heritability of wing morphology is high, and we suggest that this high heritability could be maintained, in part, by antagonistic pleiotropy. In dimorphic species, the winged morph is generally considered to be the migrant form. However, there are significant correlations, both within and among species, between the proportion of winged individuals, the proportion of winged individuals with functional flight muscles, and the flight propensity of those individuals. This suggests that the proportion of winged individuals and the propensity of the winged morph to migrate are intimately connected at both the physiological and population level. Therefore, the study of the evolution of wing dimorphism is important not only in its own right but also as a model of how migratory propensity evolves in monomorphically winged species.