The critical role of microorganisms in induction and perpetuation of pulpal and periapical diseases have clearly been indicated. Considering the complex anatomy of root canal pulp space complete elimination of bacteria from the root canal system by mechanical instrumentation alone is unlikely. Therefore, using root canal irrigants and medicaments is necessary to kill microorganisms and to remove tissues from the root canal system. Sodium hypochlorite is the most popular root canal irrigation solution with excellent antimicrobial activity and tissue dissolving ability. The purpose of writing this book was to review different aspects of sodium hypochlorite in endodontics.