Dogs adopted from shelters are often referred to as shelter dogs or pound puppies; dogs adopted from rescue organizations are often called rescued dogs (not to be confused with search and rescue dogs). Shelters have put together informational websites to help the public choose the right dog for their family.

Pets are taken to animal shelters for many reasons.

People deal with their unwanted pets in many ways. Some people have the pet euthanized (also known as putting it down or putting it to sleep), although many veterinarians do not consider this to be an ethical use of their resources for young and healthy animals, while others argue that euthanasia is a more humane option than leaving a pet in a cage for very long periods of time. Other people simply release the pet into the wild or otherwise abandon it, with the expectation that it will be able to take care of itself or that it will be found and adopted. More often, these pets succumb to hunger, weather, traffic, or common and treatable health problems. Some people euthanize pets because of terminal illnesses or injuries, while others even do it for common health problems that they cannot, or will not, pay for treating. More responsible owners will take the pet to a shelter, or call a rescue organization, where it will be cared for properly until a home can be found. Homes cannot always be found, however, and euthanasia is often used for the excess animals to make room for newer pets, unless the place has a no-kill policy. The Humane Society of the United States estimates that 3-4 million dogs and cats are euthanized each year in the US because of a lack of homes.[citation needed] Animal protection advocates urge people to spay or neuter their pets and to adopt instead of buying animals in order to reduce the number of animals who have to be euthanized.

To help lower the number of animals euthanized, some shelters have developed a no-kill policy. These shelters keep their animals as long as it takes to find them new homes. City shelters rarely have this policy because of the large number of animals they receive. No-kill shelters are usually run by groups that have volunteers or individuals with enough space to foster pets until a permanent home can be found. However, many of these groups and individuals have a finite number of spaces available. This means they will not take in new animals unless a space opens up, although they will often take back pets that they have adopted out previously.

The central issue in adoption is whether a new owner can provide a safe, secure, permanent home for the pet. Responsible shelters, pounds, and rescue organizations refuse to supply animals to people whom they judge unable to supply the animal with a suitable home.

Sometimes, a new owner may face training or behavioral challenges with a pet who has been neglected, abused, or left untrained. In the vast majority of cases, patience, training, and consistancy of care will help the pet overcome its past.

One problem shelters are fighting to overcome is what they term "Big Black Dog syndrome". Big black dogs (BBDs) are consistently the hardest dogs to place — even if they’re friendly, well trained, and in perfect health. This may be due to a number of factors, including fear stigma against certain breed types, attraction to ads and the fact that black dogs often do not photograph as well as lighter coated ones, and the fact that black dogs are often portrayed as aggressive in film and on television. Organizations have started campaigns to educate the public about BBD syndrome.