A root canal is the removal of a nerve from the tooth. Each tooth has at least one nerve that runs through the tooth and connect to the gum. The nerve is the life source of the tooth. If the nerve becomes infected or exposed (due to decay or structural damage), it can be very painful. A root canal removes the nerve and fills the nerve cavity with composite material. While the removal of the nerve relieves the patient of pain, it also removes the life source of the tooth. A tooth that has a root canal done on it almost always needs a crown to be places as well to reinforce the strength of the tooth.