A no-kill shelter is an animal shelter that does not kill healthy or treatable animals based on time limits or capacity, reserving euthanasia for terminally ill animals, animals suffering poor quality of life, or those considered dangerous to public safety. Some no-kill shelters will commit to not killing any animals at all, under any circumstance, except as required by law. A no-kill shelter uses many strategies to promote shelter animals; to expand its resources using volunteers, housing and medical protocols; and to work actively to lower the number of homeless animals entering the shelter system. Up to ten percent of animals could be killed in a no-kill shelter and still be considered a no-kill shelter.

A no-kill shelter is a shelter that saves healthy, treatable and rehabilitatable animals and reduces their euthanasia rates by screening and selecting the animals they bring into their care, known as a limited admission shelter. As a benchmark, at least 90% of the animals entering the shelter are expected to be saved. The save rate must be based on all animals entering the shelter.

Some shelters claim they are no kill when they save all "adoptable" animals, but continue to kill many healthy, treatable, or rehabilitatable animals, such as feral cats. No kill advocate Nathan Winograd states that a Los Angeles animal shelter "was claiming to be saving almost all 'adoptable' animals even while it was killing half the dogs and 80% of all cats. A shelter does not achieve No Kill by calling animals 'unadoptable' before killing them; it achieves No Kill by actually saving their lives."

India has the world's oldest no-kill traditions. The earliest instances of high volume spaying/neutering of stray dogs were done in India. In 1994, the city of Mumbai agreed to handle dog control on a no-kill basis. In 1998, the Indian government announced the goal of the whole country becoming no-kill by 2005. At that time, cities such as Delhi, Chennai and Jaipur had already adopted no-kill.

Italy has outlawed the euthanasia of healthy companion animals since 1991 and controls stray populations through trap, neuter and return programs. A compilation of 10 years' worth of data on feral cat colonies in Rome has shown that although trap-neuter-return decreased the cat population, pet abandonment was a significant problem. Dog attacks on Italian citizens and tourists have been blamed on a lack of enforcement of animal control laws in the country.

The Polish law prohibits the killing of animals, therefore all Polish shelters are no-kill. The only exception are sick animals, especially those suffering or endangering by their sickness other animals. Blind litters are also excluded from the legal protection of life, it is recognized that due to their dependence, they can be killed. Such legislation has led to a situation in which some shelters conceal the possession of dogs from their owners in order to increase profits - the stay of dogs in shelters is paid by the government at a daily rate.

In Portugal, euthanasia is practiced at publicly owned kennels although several different associations actively shelter strays. Among those is Patas Errantes, a non-profit private organization founded in 2006 which practices a policy of taking dogs off the street, vaccinating and sterilizing them, and either returning them to the streets or finding them new owners. Liga Portuguesa dos Direitos do Animal, a public utility state-recognized organization founded in 1981, is also quite active in animal sterilization and fights for no-kill. Sintra town kennel is noted for having ceased euthanasia practices in their kennel. However, the shelter admits that during the holidays, it is so overwhelmed with unwanted and discarded animals that euthanasia is required for humane reasons.

Pets Alive is an American no-kill shelter, based in Middletown, NY, who used to operate in Puerto Rico, rescuing canines from "Dead Dog Beach", where people leave their strays. It shipped some of the dogs to their Middletown, Orange County, New York location in New York.