Most people will have two sets of teeth in their lifetime, 20 primary teeth (a.k.a. "baby teeth" or "deciduous teeth") and up to 32 permanent teeth.

As the primary teeth get ready to erupt into the mouth a process called "teething" occurs as the gums get hard, red and swollen. This process can cause discomfort, excessive drooling, runny noses, low-grade temperatures and often increases a child's crankiness. Each child's experience is likely to show differences, so there is an average range within which tooth eruption occurs. A child who breathes second-hand smoke from one or both parents is likely to have a delay in the eruption of his/her teeth by 4 to 5 months. It is rare that any teeth are visible at birth, but on occasion a baby is born with 1-2 teeth.

The first teeth to arrive are the two lower front teeth (deciduous central incisors) when the child is between 6-8 months old. Their upper counterparts (the teeth on top) will arrive soon after the lowers. Between ages 7-10 months, the front four teeth will appear both on bottom and on top. After these 8 teeth are in the mouth, the next to appear are a child's first molars around the ages of 12-16 months. At ages 16-20 months the child's canine teeth will erupt between the front teeth and the first molars, two on top and two below. The last primary teeth will appear when the child is around 2-2.5 years old and will erupt behind the child's first molars.