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Introduction: Root canal instrumentation is a challenging procedure, due to different factors. These factors can be related to tooth anatomy, operator skills and experience and instruments used during the treatment. Certain areas regarding operator experience and root canal instrumentation in the endodontics literature are still not fully understood and are in need of further research. Aims: The aim of this thesis was to review the available literature on root canal instrumentation. The two main aspects investigated in this thesis, are operator experience and its effect on procedural errors during root canal instrumentation and the ability of rotary nickel titanium instruments in achieving three-dimensional instrumentation of the root canal system using modern instruments that can conserve the tooth structure. The objective of the 1st study is to investigate the procedural errors created and time efficiency of modern engine driven rotary endodontic file systems used by inexperienced users. The objective of the 2nd study was to investigate the percentage of root canal surface instrumentation and amount of dentine preservation achieved by a recently introduced endodontic file system claimed to achieve higher percentage instrumentation of root canal walls, whilst conserving tooth structure. A second objective of this study was to investigate the instrumentation effect of XP-endo finisher (XPF) rotary NiTi file at the end of preparation sequence on the percentage of root canal wall instrumentation, following the use of XP-endo Shaper (XPS) file system compared with ProTaper Next (PTN) file system. Methodology: The evidence present in the literature regarding the effect of the operator experience on the amount of procedural errors created during root canal preparation and the efficacy of instrumentation of the root canal walls was reviewed and is presented in Chapter 2. The literature was searched using google scholar, PubMed and web of Science and a narrative review was completed. An in vitro crossover randomised double blinded trial, using ProTaper Universal (PTU) or ProTaper Next (PTN) rotary nickel titanium (NiTi) files, to prepare simulated root canals by undergraduate dental students was conducted at the school of dentistry at University of Liverpool and is presented in Chapter 3. In vitro randomised single blinded trial was conducted to investigate the ability of instrumentation and conservation of tooth structure of two file systems XP-endo Shaper (XPS) rotary NiTi file and ProTaper Next rotary (PTN) NiTi file in 24 extracted mandibular molars, using Micro Computed Tomography (µCT) imaging and three dimensional analysis. Results: The literature revealed some gaps in knowledge regarding the effect of operator experience on the number of procedural errors produced during root canal preparation with some rotary file systems. There was a lack of evidence of recently introduced rotary file systems and its ability to instrument the root canal system. The 1st study, showed that the PTN file system was better than the PTU in producing successful preparations in a shorter time than PTU in undergraduate hands. The XPS file system demonstrated better ability to instrument the canal walls, with a higher percentage canal wall contact compared with the PTN. XPS files were more conservative of the root dentine than PTN. XPF also showed improvement in the percentage of canal wall instrumentation as a finisher file at the end of the preparation without removing a significant amount of root dentine. Conclusions: In the hands of novice operators, PTN showed a lower incidence of procedural errors and better time efficacy during instrumentation of simulated canals compared with PTU.The XPS file system achieved high percentage of root canal wall instrumentation whilst preserving root canal dentine. Using XPF file as a finisher file after any rotary file system improves the percentage of mechanical instrumentation without a significant effect on the amount of dentine removed.