The American Kestrel (Falco sparverius) is a common North American falcon. As a result of a number of characteristics, including small size, annual reproduction, ability to be housed in large numbers, and ease of breeding in captivity, these falcons have been used as wildlife models for over four decades. Kestrels are best known for their use in the demonstration that 1,1,1-trichloro-2,2-bis(p-chlorophenyl)ethane (DDT), and 1,1-dichloro-2,2-bis(p-chlorophenyl)ethylene (DDE) caused eggshell thinning and reproductive failure in birds, and their use in the amelioration of captive propagation programs for the Peregrine Falcon (F. peregrinus). Since that time, kestrels have continued to play a prominent role in wildlife research, particularly in the field of toxicology, where they remain the only raptor species to be bred in sufficient numbers in captivity for statistically significant research. Kestrels have also been used for studies of avian biology, physiology, nutrition, and behavior. In recent years, concerns have been raised about factors that might affect the future use of captive kestrels as wildlife models, namely the declines in wild kestrel populations and the possible deleterious effects of long-term captive breeding, which could have implications for the comparability of wild and captive populations.