It is fairly rare that there are extra teeth but is does occur. One of the most common areas that this occurs is at the second from the centre teeth on the upper teeth. When there are extra teeth it is simply due to genetics and it will cause severe crowding if such an extra tooth is allowed to stay in the mouth. Normally an extra tooth will be removed as soon as is practical. Another common are for extra teeth is at the wisdom teeth (third molars).
Missing teeth are fairly common and often cause considerable difficulty in orthodontic treatment. Teeth can be missing due to loss of the tooth from trauma or decay or teeth can be congenitally missing. The common areas where teeth are congenitally missing are the second from the centre upper teeth (upper lateral incisors) and the lower teeth just before the molars (lower second premolars).
Sometimes the baby teeth can remain in the place of the missing adult teeth but often the baby teeth will fall out and the adjacent adult teeth shift into the gap. The decision must be made as to whether the space of the missing teeth should be maintained for future replacement with an implant or if the space can be closed with orthodontic treatment while providing a normal and functional bite.