The aim of this study was to systematically review the histological evidence of the neo-formed tissues inside the root canals of human teeth having previously received regenerative endodontic treatment. An electronic research was performed in the MEDLINE, Web of Science, Scopus and Cochrane Library databases using controlled vocabulary. The retrieved studies were screened by two reviewers according to the predetermined inclusion and extrusion criteria and were full-text-evaluated. Research resulted in 160 studies. Among them, twelve fitted the inclusion criteria and were critically appraised. The tissues formed in the root canals of immature human teeth treated with REP indicate repair or a combination of repair and regeneration. Pulp remnants and healthy periapical tissues seem to improve regeneration. The level of available evidence was low. Further clinical studies are needed in order to establish the appropriate treatment protocol related to the pretreatment status of the dental pulp and the periapical tissues.