Pet adoption usually refers to the process of taking guardianship of and responsibility for a pet that a previous owner has abandoned or otherwise abdicated responsibility for. Common sources are

• Animal shelters, in the case of dogs also known as dog pounds
• Rescue groups (e.g. the notable St. Louis area-based group "Paws for Consideration")
• Pets out of captivity without identification, and which remain unclaimed by any owner
• Advertisements placed by individuals who are trying to find a new home for their pet
• Pets that have been abused or neglected and have been confiscated from the offending owner
• A fast growing source is online pet adoption through not-for-profit websites. Public service sites such as these have searchable databases of pets, maintained by thousands of animals shelters, and are searchable by the public.

Dogs adopted from shelters are often referred to as shelter dogs or pound puppies; dogs adopted from are often called rescue (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.

• Death: Owner dies and no one in the family wants to (or can) keep the pet.
• Changed circumstances: Financial or living arrangements change drastically and people feel that they can no longer provide an appropriate home for the pet. This might also include someone having to move to a new living situation where pets are not allowed.
• Second thoughts: A pet was purchased after the spur of the moment decision or as a gift (frequently for Christmas). Some time afterward, owners discover that caring for the pet is much more work than expected, or requires more space or exercise than they are prepared to give.
• Lost pet: Pet leaves home or cannot find its way back, and the owner does not succeed in finding it.
• Health: Severe health problems make it impossible to have a dog in the house or impossible for the owner to care for the dog.
• Practice babies: Shelters use this term for animals that have been adopted by couples, and who are then abandoned when the couple splits up, or when a real baby comes along and they no longer have the time or inclination to care for their pet.
• Moving across borders:

  People leave the country; quarantine laws in some countries can be traumatic to pets and owners, so to avoid the stress, the pet is surrendered to an animal shelter.

• Allergies: Many owners claim to have developed allergies to their pets, or that their children have developed allergies to their pets.

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 abandoning a pet someplace to fend for themselves. 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 new home can be found for the animal. In shelters, euthanasia is often used to make room for newer animals, 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. 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 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 the adoption of a pet is whether a new owner can provide a safe, secure, permanent home for the pet. Many shelters and pounds cannot supply the animal with a suitable home. A new owner might also face a pet who has been neglected or abused. In those cases, the owner must be extra patient with the animal and provide it with the right care to help the pet overcome the past.