Teeth are held in the jaws by their roots. Front teeth normally have one root, but teeth further back have more. At the core of each tooth is a soft mass of tissue called the pulp. Tooth decay or injury can destroy the living pulp and make it is more prone to infection. This can lead to an abscess and toothache.
Root fillings (also known as root canal treatment or endodontics) are carried out to remove damaged or dead pulp. The space left can then be filled to prevent any further infection getting in. An x-ray can show the number and shape of the root canals, and also signs of infection (an abscess) in the surrounding bone.
The diagram shows a decayed tooth with a single root being filled.
You will usually be given a local anaesthetic to numb the tooth. Then an opening is made through the top of the tooth, down into the pulp, using a drill. The dentist then uses narrow files to remove the dead pulp from the core of the tooth and from the root canal.
At this point, the dentist may put in a temporary filling (not shown) and possibly also give you antibiotics if any infection has spread beyond the tooth. If so, you will have to return at a later date, once symptoms have settled, so the dentist can complete the treatment.