Bowel preparation is essential for successful colonoscopy examination, and the most important factor is the bowel preparation agent used. However, selection of a bowel preparation agent invariably involves compromise. Originally, bowel preparation was performed for radiologic and surgical purposes, when the process involved dietary limitations, cathartics, and enemas, which had many side effects. Development of polyethylene glycol (PEG) solution led to substantive advancement of bowel preparation; however, despite its effectiveness and safety, the large volume involved, and its salty taste and unpleasant odor reduce compliance. Accordingly, modified PEG solutions requiring consumption of lower volumes and sulfate-free solutions were developed. Aqueous sodium phosphate is more effective and better tolerated than PEG solutions; however, fatal complications have occurred due to water and electrolyte shifts. Therefore, aqueous sodium phosphate was withdrawn by the US Food and Drug Administration, and currently, only sodium phosphate tablets remain available. In addition, oral sulfate solution and sodium picosulfate/magnesium citrate are also available, and various studies have reported on adjunctive preparations, such as hyperosmolar or stimulant laxatives, antiemetics, and prokinetics, which are now in various stages of development.