Non-technical skills (NTS) are crucial to effective team working in endoscopy. Training in NTS has been shown to improve team performance and patient outcomes. As such, NTS training and assessment are now considered essential components of the endoscopy quality assurance process. Across the literature, other specialties have achieved this through development of behavioural marker systems (BMS). BMS provide a framework for assessing, training and measuring the NTS relevant to healthcare individuals and team. This article describes the development and impact of a novel BMS for endoscopy: the endoscopic non-technical skills (ENTS) system. The initial NTS taxonomy for endoscopy was created through a combination of literature review, staff focus groups and semi-structured interviews, incorporating the critical decision method. Framework analysis was conducted with three individual coders and generated a skills list which formed the preliminary taxonomy. Video observation of Bowel Cancer Screening endoscopists was used to identify exemplar behaviours which were mapped to relevant skills in the NTS taxonomy. Behavioural descriptors, derived from video data, were added to form the basis of the ENTS system. A taxonomy of 33 skills in 14 separate categories were identified through framework analysis. Following video analysis and behaviour mapping, 4 overarching categories and 13 behavioural elements were identified which formed the ENTS framework. The endoscopy (directly observed procedural skills) 4-point rating scale was added to create the final ENTS system. Since its development in 2010, the ENTS system has been validated in the assessment of endoscopy for trainees nationally. ENTS informs a number of training initiatives, including a national strategy to improve NTS for all endoscopists. The ENTS system is a clinically relevant tool, validated for use in trainee assessment. The use of ENTS will be important to the future of training and quality assurance in endoscopy.