This article discusses the place of symptom detection, endoscopy, and fecal occult blood testing in population screening for colorectal cancer. There is now considerable evidence that screening the population over the age of 50 years for occult blood in the feces will result in an increased yield of tumors localized to the bowel at the time of surgical treatment. These tumors also have other favorable prognostic features and it is likely that the prognosis of this group will be better than the prognosis of patients presenting with symptoms in the usual way. Because of the biases that result from the selection and detection of tumors in screening studies, the mortality results of the control trials now underway must be awaited until it is known whether population screening is of real value. In chemical fecal occult blood screening tests, a compromise has to be made between sensitivity and specificity. The fecal occult blood test most widely used and the one that has been subjected to the most evaluation in screening studies is the guaiac-based slide test, Hemoccult. The predictive value of a positive test for invasive cancer is 11-17%, and for adenomas, 36-41%. This specificity is achieved at a loss of sensitivity, the interval cancer rate reported in screening studies being over 20%. Newly developed immunological techniques appear to be more sensitive and specific, but require further evaluation in population screening studies.