The Department of Agriculture Secretary Mike Espy (defendant) promulgated a regulation that excluded birds, aquatic animals, rats, and mice from the definition of an animal. Espy was sued by Patricia Knowles, a former researcher who experimented on rats and mice; William Strauss, who was responsible for ensuring that certain facilities complied with legislation and regulations concerning the proper treatment of animals; and the Animal Legal Defense Fund and the Humane Society of the United States (organizations), which sought to promote the humane treatment of animals (plaintiffs). Knowles, Strauss, and the organizations claimed that Espy had violated the Animal Welfare Act by excluding birds, aquatic animals, rats, and mice from the definition of animals because the exclusion allowed for such creatures to be treated inhumanely. To establish standing, Knowles claimed that she would eventually return to her research and that her research was likely to suffer because research institutions no longer had to treat rats and mice humanely. Strauss claimed that he had standing because he would be unable to ensure that facilities with birds, aquatic animals, rats, and mice complied with the Animal Welfare Act because there was no guidance on proper treatment standards for creatures not covered by the animal definition. The organizations claimed that they had standing to sue because of their interest in promoting the humane treatment of animals. The district court found that standing was proper and granted the organizations’ motion for summary judgment. The decision was appealed. On appeal, Espy did not challenge the ruling on standing. However, the appellate court reviewed the issue of standing, explaining that standing was a constitutional requirement.