Most dogs are still considered puppies during the first two years of their lives, in both mind and body. During this time, they go through several different stages of development.

Puppies should ideally stay with their littermates for at least the first 12 weeks of their lives. There are many things that a puppy learns from his mother and littermates during the first eight weeks, skills which, if not acquired during that time, may be lost forever. Many of these are social skills. In fact, puppies of well-socialized mothers are more likely to be well-socialized themselves as they will follow their mother's lead when it comes to interacting with people. From their littermates, puppies learn other social skills like appropriate play and limits when it comes to mouthing and biting.

Below is a general guideline for the stages of development that a puppy goes through:

Age 0 to 2 weeks: Neonatal. During this time, puppies are most influenced by their mother. In addition, at birth, the senses of touch and taste are present.

Age 2 to 4 weeks: Transitional. During this time, puppies are most influenced by both their mother and littermates. Their eyes are open, their senses of hearing and smell are developing, their teeth are beginning to erupt, and they are starting to stand, walk a bit, wag their tail and bark. Their sense of sight becomes well-developed by around four or five weeks of age.

Age 3 to 12 weeks: Socialization. During this time, it's very important that puppies be given many chances to meet people and other dogs. Their littermates become the most influential factor in their life by four to six weeks, at which time they're also learning about being a dog. Between the ages of four to twelve weeks, both littermates and people become the greatest influence to a puppy, at which time they're learning social skills, how to play, physical coordination, inhibited bite, and social structure and ranking. Between the ages of three to five weeks, puppies begin to become aware of their surrounding, including their companions and relationships. Curiosity and a need to explore new experiences occurs between five and seven weeks of age, during which time they need to have positive experiences with people. A puppy's physical skills and coordination are being refined and the full use of the senses occurs by seven to nine weeks, while real fear, often to normal objects and experiences, is experienced by eight to ten weeks, at which time positive training should be used. By nine to twelve weeks of age, puppies begin to focus on people, making it a good time to start training.

Age 3 to 6 months: Ranking. During this time, puppies are most influenced by their ‘littermates', which now includes human, and they begin to understand and put to use ranking within the pack. They are also teething at this time. Another fear stage is experienced around four months of age. 

Age 6 to 18 months: Adolescence. During this time, human and dog ‘pack' members become the most influential factor to a puppy. Sexual behavior begins if not spayed or neutered, and puppies begin to further explore dominance and may challenge humans. A second chewing phase is also experienced around the age of seven to nine months.