The incidence of colorectal cancer (CRC) has been increasing during the past decades, and the lifetime risk for CRC in industrialised countries is about 5%. CRC is a good candidate for screening, because it is a disease with high prevalence, has recognised precursors, and early treatment is beneficial. This paper outlines the evidence for efficacy from randomised trials for the most commonly used CRC screening tests to reduce CRC incidence and mortality in the average-risk population. Four randomised trials have investigated the effect of guaiac-based fecal occult blood screening on CRC mortality, with a combined CRC mortality risk reduction of 15-17% in an intention-to-screen analysis, and 25% for those people who attended screening. Flexible sigmoidoscopy screening has been evaluated in three randomised trials. The observed reduction in CRC incidence varied between 23 and 80%, and between 27 and 67% for CRC mortality, respectively (intention-to-screen analyses) in the trials with long follow-up time. No randomised trials exist in other CRC screening tools, included colonoscopy screening. FOBT and flexible sigmoidoscopy are the two CRC screening methods which have been tested in randomised trials and shown to reduce CRC mortality. These tests can be recommended for CRC screening.