There is a narrow margin between success and failure of root canal treatment due to the complex anatomy of the root canal system. Visualisation of the complexity of the root canal system is offered by the impressive anatomical images taken by Walter Hess, an anatomist from Zurich, at the end of the last century. The endodontium includes, in total, the main communicating root canals, numberless small lateral canals and several ramifications. In the dental practice, the location, shaping and sealing of root canals which are closely related and have ramifications, represent a special challenge for the clincian and is very often an almost insolvable task. This highlights how the knowledge of anatomy decides the technical parameters of the root canal treatment and, at the same time, it decides upon the probability of success. Therefore, the following overview presents explanations concerning the incidence and classifications of ramifications and lateral canals in the literature and their clinical significance, as well as the resulting consequences and the treatment strategies illustrated by several clinical cases. Keywords: access cavity, branched root canal systems, dental operating microscope, intracanal endodontic diagnostics, tooth anatomy