The American Society for the Prevention of Cruelty to Animals (ASPCA) estimates that there may be anywhere between 4-6,000 animal shelters in the United States. There is no federal oversight of these shelters. While some states do have regulations and guidelines governing the operation of municipal shelters, many are privately funded and operated with no oversight. Animal control programs were originally designed in the 1900s to handle the growing concerns about public health and safety issues posed by stray dogs, including increasing numbers of dog bites, automobile accidents, rabies concerns etc. Local ordinances governing licensing, vaccinations, leashing dogs in public, noise and nuisance issues were enacted to make owners more responsible. When those laws are broken, animal control steps in to enforce them. But animal sheltering today entails much more than enforcing the animal control laws. Animals are housed in shelters for a variety of reasons, but regardless of the reason, in order to implement appropriate and humane standards of care and provide for quality of life, the input of the veterinary community is increasingly being sought. In addition to providing necessary shelter, food, water and basic veterinary care, shelters are expected to implement programs that enhance wellness, alleviate suffering and promote the health and well-being of the individual animal as well as the population as a whole. While shelters in the past often utilized euthanasia to manage disease and population numbers, this method is increasingly being rejected as a primary means of animal control in many communities, resulting in an increased demand for veterinary services.

Shelters may be generally categorized as municipal or private. Knowledge of their governance, mission and funding is essential for design of effective health care programs. Municipal animal shelters are often primarily concerned with animal control. They do not turn animals away and are thus often referred to as "open admission". Animal intake can occur for a variety of reasons. The National Council on Pet Population Study and Policy (NCPPSP) conducted studies to determine owner reasons for relinquishment, but animals are held in shelters that were not voluntarily relinquished. In addition to stray and abandoned animals, some animals may be housed because their owners are temporarily homeless due to eviction or fires, the owner may be deceased or in the hospital and there is no one else to care for the animal, or they may have been seized because of cruelty complaints against their owners. Shelter stays may be long or short term, requiring different housing arrangements, enrichment procedures and medical protocols. Municipal shelters can range from being marginally funded by the city and managed by the police department with minimal standards of care, to a few being well funded with the highest standards of care developed by animal care professionals. In some cases, a privately operated Society for the Prevention of Cruelty to Animals (SPCA) or humane society may accept the contract for animal control and receive private as well as municipal funding. Animal shelter funding is often limited regardless of whether it is private or municipal, resulting in limited options for the implementation of optimal health care protocols and operational guidelines. Internal policies, resource limitations and local statutes have a tremendous impact on veterinary decisions and policies aimed toward the provision of care.

Some private shelters are run by the local society for the prevention of cruelty to animals (SPCA), or the local humane society with no municipal funding. They are usually limited admission although some may accept all animals. While they are also bound by some legal restrictions imposed by local and state government, their internal policies will affect many of the decisions the veterinarian must make, especially in the face of disease outbreaks and overcrowding. Some private shelters are also known as "no-kill" shelters, which represent a growing trend in animal sheltering. Many of these shelters are operated by not- for -profit organizations. Their internal policies generally state that they will not euthanize "adoptable" animals, thus eliminating the use of euthanasia to control overcrowding or manage disease outbreaks. They maintain the right to refuse to admit animals when they run out of space, thus effectively eliminating them from performing many municipal animal control functions. Adoptability is defined by each agency and may be affected by many factors, including their financial ability to isolate sick animals and treat disease, the expertise of the staff in performing behavior evaluations and treatments, population numbers, etc.