When decay in a tooth is so extensive that it goes through the inside layer of your tooth to the area where the nerve and blood vessel of your tooth is housed root canal therapy needs to be performed. Root canal therapy is the process of removing the infected nerve and blood tissue, bacterial debris and by products, and reshaping the canals and sealing them off with with a rubber material called Gutta Percha. This is done to help your bodies defense mechanism fight off infection in the tooth where it's hard for the white blood cells to reach, and with the hope that your body can fight off any infection that escaped the tooth from the root end. Despite it's notorious reputation for pain with proper anesthesia, and modern advancements in this area root canal therapy should be as comfortable as getting a regular dental filling.