I research _ working length Determining working length, or how to locate the apical terminus (Part I) Authors_Prof Vladimir Ivanovic & Dr Katarina Beljic-Ivanovic, Serbia Fig. 1_The apical foramen almost never coincides with the principal axis of the root. Fig. 2_The anatomical foramen is not always located at the anatomical apex. Fig. 1 Fig. 2 _Introduction _Determining working length (WL) is one of the crucial aspects in successful endodontic treatment. However, major controversy exists regarding where to locate the apical end point of the root canal preparation and obturation. The ongoing debate centres on different concepts of shaping and cleaning the apical portion of the root canal, and whether to complete manipulation within the dentinal walls to the physiologic foramen or to extend into the cemental cone close to the anatomical foramen. Several colleagues contributed their research results to this article. They are Prof Mirjana Vujaskovc, Dr Katarina Beljic-Ivanovic, Dr Jugoslav Ilic and Dr Ivana Bosnjak from the Faculty of Dental Medicine at the University of Belgrade; Prof Joshua Moshonov from the Hebrew University of Jerusalem, who incorporated parts of his own research and provided laboratory testing; Dr Julian Webber, who supplied materials and gave useful advice; and Prof Paul Dummer from Cardiff University, who contributed valuable suggestions and instructions for our in vivo study. This article is based on my recent lecture at the European Society of Endodontology (ESE) congress in Edinburgh for which I consulted numerous papers and books. I wish to point out a few that have directly influenced my own standpoints and work over the years and finally directed much of this article through their philosophy and conception: _Ricucci D, Langeland K: Apical Limit of Root Canal Instrumentation and Obturation, Part 1: Literature Review. International Endodontic Journal 1998; 31(6):384–393. _Ricucci D, Langeland K: Apical Limit of Root Canal Instrumentation and Obturation, Part 2: A histological study. International Endodontic Journal 1998; 31(6):384–409. _Wu M, Wesselink P, Walton R: Apical Terminus Location of Root Canal Treatment Procedures. Oral Surgery, Oral Medicine, Oral Pathology, Oral Radiology and Endodontics 2000; 89(1):99–103. _Fava LRG, Siqueira JF: Considerations in Working Length Determination. Endodontic Practice 2000; 3(5):22–33. Figs. 3a & b_The anatomical foramen is not located at the radiographic apex, and canal instruments exit through the foramen in various angulations. Fig. 3a Fig. 3b 30 I roots 4_ 2009