The mandibular first molar is first permanent tooth to erupt in theoral cavity. It bear maximum masticatory load. it is frequenty affected by caries and require root canal treatment. This book is focused on morphological variation in the root and root canal system of mandibular first molar. Various case reports are also included in this book to help readers to understand root canal variations. This book will be a guidence for clinicians who practises endodontics as well as post graduates of endodontics.