Bowel preparation is a critical part of colonoscopy and has the potential to reduce deaths from cancer with many countries introducing national screening programs. For colonoscopy to be successful, it is necessary for the endoscopist to detect and remove premalignant adenomatous polyps. Some adenomatous and sessile serrated polyps can be flat and subtle. Therefore, the effectiveness of colonoscopy is dependent on the identification of subtle early changes. Although there has been considerable discussion around colonoscope resolution and image enhancement techniques, none of these developments are of value if the colon is not completely clean, and it is established that poor bowel preparation leads to lesions being missed. Therefore, the importance of high-quality bowel preparation cannot be underestimated. In particular, although there are many preparations and regimens available, patient compliance with bowel cleansing can be a significant issue, with one in seven patients showing a lack of compliance with a standard split-dose regimen. Much of the controversy surrounding bowl cleansing has centered around how the dose should be given. Evidence has emerged from several well-conducted studies suggesting that split-dose preparation is superior to single-dose delivery in achieving adequate bowel cleansing. Such regimens have involved the patient taking some bowel preparation the day before the procedure with further medication given on the morning of colonoscopy. Furthermore, there are data emerging that suggest that the final dose needs to be given as close to the procedure as possible, ideally within 5 h of colonoscopy.