Many passerine birds and their relatives utter flight calls, species-specific vocalizations given primarily during sustained flight, especially during migration. References to flight calls appeared in the ornithological literature as early as the 1890s, but some of the most basic features of these calls remain poorly known, including their functions, origins, ontogeny, distances over which they are used, and how much individual variation exists in the characters of the calls and their rates. With improved knowledge of these vocalizations, flight calls possibly will have a variety of applications. Identifying their function could illuminate how birds refine migration strategies during crucial decision-making periods. Because flight calls are relatively simple vocalizations, compared with many others that birds use, they provide useful characters for future evolutionary and comparative analyses. Monitoring flight calls can be a powerful method for studying nocturnal migration. However, such applications require more detailed knowledge of flight-calling behavior. Here, I summarize the available information on flight calls and highlight areas where future research may improve and expand this knowledge.