This is an update of a review first published in 2007. Root canal treatment, or endodontic treatment, is a common procedure in dentistry. The main reasons that root canal treatment are needed are persistent inflammation of the dental pulp (pulpitis) and death of the dental pulp (dead or non-vital tooth) caused by tooth decay, cracks or chips, or other accidental damage to teeth. Root canal treatment is considered successful when there are no symptoms, for example pain, and when x-rays show no signs of damage to bone and other supporting tissues of the tooth. The success of root canal treatment depends on the preoperative condition of the tooth, as well as the endodontic procedures used. There are two approaches commonly used. In the first, the root canal treatment is performed in multiple visits to the dental clinic, and a dressing is placed in the tooth between appointments to avoid the build up of bacteria that may cause infections. In the second approach, the treatment is performed in one visit to the dental clinic. Which approach leads to less postoperative complications?