Although the statistics vary, estimates suggest that three to four million animals are euthanized in U.S. animal shelters every year.

On a hot Sunday morning in July, J.C. Burcham, DVM, and a colleague neuter 79 cats at a local animal welfare organization. Dr. Burcham, who practices in a large veterinary hospital in Olathe, Kan., knows firsthand about relinquishment and euthanasia.

Misbehaving is the most dangerous thing a companion animal can do.

Shelters can adopt out only so many animals, says Kate Hurley, DVM, MPVM, director of the Koret Shelter Medicine Program at the University of California, Davis. So the biggest impact on euthanasia numbers will be on the intake side of the equation, not the adoption side.

Although it is obvious to veterinarians that a Border collie and a 96-year-old woman likely make a poor pet-owner match, it may not be obvious to a potential owner who has never been around Border collies. Indeed, one reason healthy animals wind up in shelters, says Gail Golab, PhD, DVM, American Veterinary Medical Association (AVMA) interim director for animal welfare, is because people "acquire a pet with an expectation the pet doesn't fulfill."

One yellow tabby named Darwin will not soon be forgotten by anyone who knows his story. In April 2004, this 9-lb cat was presented DOA to Brooklyn, N.Y., emergency veterinarian Brett Levitzke. Dr. Levitzke knew immediately that Darwin had died as a result of trauma. "I took the woman who brought Darwin in aside and asked her what had happened," he says. "She said her daughter's fiancé had beaten the cat. I told her that I take this very seriously and that I would get law enforcement involved. She said, 'OK, I want this guy prosecuted.'"