Colonoscopy is a commonly performed procedure for the diagnosis and treatment of a wide range of conditions and symptoms and for the screening and surveillance of colorectal neoplasia. Although up to 33% of patients report at least one minor, transient GI symptom after colonoscopy, serious complications are uncommon. In a 2008 systematic review of 12 studies totaling 57,742 colonoscopies performed for average risk screening, the pooled overall serious adverse event rate was 2.8 per 1000 procedures. The risk of some complications may be higher if the colonoscopy is performed for an indication other than screening. The colorectal cancer miss rate of colonoscopy has been reported to be as high as 6% and the miss rate for adenomas larger than 1 cm is 12% to 17%. Although missed lesions are considered a poor outcome of colonoscopy, they are not a complication of the procedure per se and will not be discussed further in this document. Over 85% of the serious colonoscopy complications are reported in patients undergoing colonoscopy with polypectomy. An analysis of Canadian administrative data, including over 97,000 colonoscopies, found that polypectomy was associated with a 7-fold increase in the risk of bleeding or perforation. However, complication data are often not stratiﬁed by whether or not polypectomy was performed. Therefore, complications of polypectomy are discussed with those of diagnostic colonoscopy. A discussion of the diagnosis and management of all complications of colonoscopy is beyond the scope of this document, although general principles are reviewed.