Pain management is one of the most important aspects of endodontic treatment. Infected root canals are the most common cause of endodontic pain. Apical extrusion of debris is a drawback of all endodontic instrumentation techniques. This can result in post-operative pain and discomfort due to inflammation of periapical tissues. It is generally accepted that crown down technique and balanced force technique cause less apical extrusion of debris and hence lesser post-operative pain. In earlier studies related to lasers, the use of lasers was limited to treating dentinal hypersensitivity, for its effects on vital pulpotomy, and for evaluating postoperative pain and swelling in patient undergoing extractions of impacted 3rd molar. Use of lasers after chemo-mechanical preparation have been shown in some studies to affect pain outcomes favourably. However, the crown down technique for NiTi has several drawbacks such as file breakage, poor dentin conservation (predisposing to vertical root fractures) and high recurring cost. According to our literature search, post-operative pain due to debris extrusion with k400 reciprocating handpiece for hand files using step back technique has never been compared with NiTi rotary instrumentation using crown down technique. Also, to the best of our knowledge, Biolase EpicX, K400 Reciprocation Handpiece, and Pro HS Niti Rotary systems have not been used in the investigations for the same. Therefore, this clinical study proposes to investigate the incidence of post-operative pain following chemo-mechanical preparation of single rooted teeth having symptomatic apical periodontitis using rotary instrumentation with Pro HS file system and engine driven reciprocating instrumentation with K-flex files along with laser irradiation (by Biolase EpicX).