The most common role of a domestic dog in the developed world currently is that of companion. Puppy socialization practices play a large role in the development of well-adjusted adult dogs that display few undesirable behaviors, and which can establish a positive, lifelong relationship with their owner. Age-appropriate socialization practices should begin within a few days of birth, and should extend well into adulthood. These practices should aim to provide exposure to many of the types of experiences, people, and objects that the dog is likely to encounter over the course of its life, in a controlled and pleasant way. Dogs that are appropriately socialized as puppies are less likely to exhibit behavioral problems as adults, including aggression and fearfulness. They are more likely to engage in positive social behaviors with humans, and can learn how to play games with humans better than dogs without proper socialization. However, the evidence in support of puppy socialization classes is less clear. There is some evidence that puppy classes positively impact adult behavior, but other studies show no clear benefit. Since socialization should begin early in a puppy's life, it is the responsibility of the breeder and the owner. Breeders can ensure that puppies are exposed to age-appropriate experiences while in the litter, and owners must ensure that the dog continues to have varied experiences throughout its life. Veterinarians are also an important part of this process, and are heavily relied upon by owners to provide information about health and behavior. Since veterinarians often see puppies during vaccinations, owners can be educated about proper socialization practices at those visits. Future research should aim to determine the minimum amounts of socialization required for a puppy, and whether there is a maximum amount, beyond which there is no benefit, or even a disadvantage.