Fillings are done to remove decay, and replace the affected tooth structure. It is called a filling because new a material fills the hole that decay left. Now days most teeth are treated with bonded tooth colored composite resin fillings. These are beautiful and work in both front and back teeth, often it is nearly impossible to see the repair (filling). Caught early enough, cavities can be treated easily and painlessly. If not treated decay will not only cause discoloration butcan lead to tooth pain and/or infection, and the tooth would need root canal treatment or to be extracted.