In the United States, oral mechanical bowel preparation (OMBP), defined as the use of an oral preparation given prior to surgery to clear fecal material from the bowel lumen, is often prescribed preoperatively for patients undergoing elective colorectal surgery. OMBP is sometimes used as a precaution in anticipation of possible iatrogenic bowel injury during abdominal and pelvic surgeries that do not entail resection of the colon or rectum (e.g., urologic or gynecologic procedures). OMBP is also routinely prescribed prior to colonoscopy to allow maximal visualization of the intraluminal bowel during the procedure. In 2009, more than 250,000 colorectal surgeries were recorded, most commonly for cancer or diverticulitis, and, in the majority of cases, in adults. In the context of colorectal surgery many have considered OMBP necessary for decreasing infectious complications, in particular by lowering anastomosis leakage rates associated with surgery. Complication rates for elective colorectal surgery range between 4 and 36 percent. A surgical site infection can substantially lengthen hospital stay from approximately 4 days to 21 days and increase costs from approximately $11,000 to $43,000. Therefore, reducing complication rates of elective colorectal surgery is an important goal. The purpose of this review was to systematically evaluate experimental and observational evidence on the benefits and adverse events associated with the use of OMBP in patients undergoing elective colorectal surgery. Patient and procedural characteristics that modify the effect of OMBP on outcomes were also reviewed. On the basis of the original topic nomination and an extensive stakeholder-driven process of topic development and refinement, we formulated the following Key Questions to guide the review: Key Question 1: How do various preoperative OMBP strategies compare with either no OMBP or with each other with respect to their effectiveness for preventing surgical or postsurgical complications? Does the effect vary by elective (a) right colon, (b) left colon, and (c) rectal surgery? Key Question 2: How do various preoperative OMBP strategies compare with either no OMBP or with each other with respect to presurgical and postsurgical adverse events? How do comparative adverse events vary (a) by OMBP strategy and (b) in subgroups of especially susceptible patients?