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: