Nearly two thirds of colorectal cancers occur in the rectum or sigmoid colon. It is the third most common cancer in resource-rich countries, accounting for about 20,000 deaths each year in the UK, and 60,000 each year in the USA.We conducted a systematic review and aimed to answer the following clinical question: What are the effects of screening for colorectal cancer? We searched: Medline, Embase, The Cochrane Library and other important databases up to November 2006 (BMJ Clinical evidence reviews are updated periodically, please check our website for the most up-to-date version of this review). We included harms alerts from relevant organisations such as the US Food and Drug Administration (FDA) and the UK Medicines and Healthcare products Regulatory Agency (MHRA).We found 19 systematic reviews, RCTs, or observational studies that met our inclusion criteria. We performed a GRADE evaluation of the quality of evidence for interventions.In this systematic review we present information relating to the effectiveness and safety of the following interventions: colonoscopy, computed tomography colography, double contrast barium enema, faecal occult blood testing, flexible sigmoidoscopy.