Education and competency assessment in gastrointestinal endoscopy is important. Concerning colonoscopy, it is not completely clear what the best way is to learn this procedure, what defines competency in colonoscopy, and which factors define a high-quality colonoscopy. The aim of this study was to determine the endoscopist-related factors that define a high-quality colonoscopy. A three-round Delphi survey among expert endoscopists was carried out. In round 1, the panel was invited to identify factors essential for a good colonoscopy. The listed factors were to be ranked during the second round. In the third round, a 5-point Likert scale was added. A reference panel was invited to assess the items as well. 14 expert endoscopists from the Netherlands were invited, of whom eight participated (57 %). A list of 30 items important for colonoscopy was formulated. After the following rounds, consensus was reached on 16 items. Validation was conducted among eight trainees and eight experienced endoscopists (response 100 %). The groups agreed on the importance of all but one factor (p = 0.001). This Delphi survey has made explicit the endoscopist-related factors that are important for optimal colonoscopy. This might provide trainers more support regarding concrete competency assessment of trainees in endoscopy.