This paper gives a brief overview of what constitutes animal hoarding. It explains the characteristics of animal hoarders and what laws prohibit the behavior.
Animal hoarding is a form of animal abuse affecting thousands of animals each year. Hoarded animals are kept in horrid conditions: deprived of socialization, denied proper care and nutrition, often living covered in their own waste and suffering from disease. Hoarded animals are often kept in various states of decay—with living animals living among (and sometimes feeding on) the remains of dead animals.
Each case of hoarding is unique, involving different species of animals, different conditions, and different hoarders. Nonetheless, the Hoarding of Animals Research Consortium (HARC) identifies the following characteristics as common in all hoarders:
- Accumulat[ion] of a large number of animals, which has overwhelmed that person’s ability to provide even minimal standards of nutrition, sanitation, and veterinary care;
- Fail[ure] to acknowledge the deteriorating condition of the animals (including disease, starvation, and even death) and the household environment (severe overcrowding, very unsanitary conditions); and
- Fail[ure] to recognize the negative effect of the collection his or her own health and well-being, and on that of other household members. 
Many people mistakenly believe that animal hoarders are merely well-meaning animal lovers who become overwhelmed by their animals. In reality, however, experts believe that hoarding is a form of mental illness and that hoarders should receive psychological evaluation and treatment.
Hoarding is generally prosecuted under state animal cruelty laws. In most states it is a misdemeanor offense, but in some states it may be a felony offense. Penalties for the offense can include fines, animal forfeiture, and jail time. Because of a high-rate of recidivism (repeating the criminal behavior) among hoarders, courts may also ban convicted hoarders from owning animals in the future or place a limit on the number of animals they may have. These orders are only effective when they are properly monitored.
In 2008, Hawaii became the first state with a specific law against animal hoarding. Legislators in some other states, such as Montana and New Mexico, have unsuccessfully attempted to pass specific laws against hoarding. Some people believe that specific anti-hoarding laws are unnecessary because hoarding can be prosecuted under animal cruelty laws. Proponents of animal hoarding laws argue that the laws are necessary in order to distinguish hoarding from other types of animal cruelty. They believe that hoarding should be distinguished in order to increase awareness and understanding of the offense that causes serious harm to multiple animals.