Animal shelters that practice euthanasia are shelters that put down animals every week. Euthanasia is the act of painlessly killing an animal, usually for humane reasons such as to end suffering from a terminal illness or injury. Animal shelters may practice euthanasia when they are overpopulated and do not have the resources to care for all of the animals in their care. Some animal shelters also practice euthanasia when they have animals that are considered to be dangerous or aggressive.