The basic prerequisites for successful endodontic treatment are firstly a comprehensive knowledge on the root canal anatomy and proper identification of all the canals, meticulous chemo-mechanical preparation followed by three dimensional obturation with hermetic seal. Failure of any of these steps may occur due to unusual tooth morphology. Usually mandibular first molars have two roots with three canals (mesiobuccal, mesiolingual & distal) but in few cases, the number of roots and canals vary. The variation of mandibular permanent 1st molar with additional distolingual root is called Radix Entomolaris (RE). This article presents seven such case reports. Also mentioned are the modifications in the access opening, canal preparation, problems encountered during the treatment, common iatrogenic errors which occur during the treatment and factors which affect the prognosis.