How to define competency in colonoscopy, how to assess it, and how much training is required are questions that experts in endoscopy have grappled with since the advent of the procedure. To describe methods to assess core endoscopy skills in trainees and learning curves for these parameters and to define competency thresholds for these skills. A prospective descriptive assessment of trainee colonoscopy performance. Mayo Clinic, Rochester, Minnesota. Gastroenterology fellows undergoing endoscopy training. From July 2007 through June 2010, fellows' core cognitive and motor colonoscopy skills were assessed by using the Mayo Colonoscopy Skills Assessment Tool (MCSAT). Average MCSAT item scores and learning curves are described. Minimal competence thresholds for each MSCAT item are established by using the contrasting groups method. Forty-one GI fellows performed 6635 colonoscopies; 4103 procedures (62%) were assessed by using the MCSAT. Average scores of 3.5 set the competency bar for each of the core skills and were reached by 275 procedures on average. Independent cecal intubation rates of 85% and cecal intubation times of 16 minutes or less were also achieved at 275 procedures on average. Limited to a single center. Learning curves for core colonoscopy skills are described. MCSAT scores of 3.5, cecal intubation rates of 85%, and intubation times of less than 16 minutes are recommended as minimal competency criteria. It takes on average 275 procedures to achieve competence in colonoscopy. This is more than previous gastroenterology training recommendations and far more than current training requirements in other specialties. Copyright © 2011 American Society for Gastrointestinal Endoscopy. Published by Mosby, Inc. All rights reserved.