The protective effect of selenium against colorectal cancer or adenoma is still a controversial issue. Although there are well-described (pato)physiological protective mechanisms of selenium against colorectal cancer, the results of the studies from 1998-2018 are inconclusive and need to be considered in the future. Neither observational nor experimental studies present consistent results. Although the Cochrane review showed that well-designed randomized clinical trials (RCTs) presented no beneficial effect of selenium supplementation on cancer incidence, well-designed RCTs confirming the protective effect of selenium supplementation against colorectal adenoma or colorectal polyp recurrence have been found in subject-related literature. In the reviewed studies, selenium concentration was measured in the blood serum/toenail or in diet. It is of great importance to highlight that blood selenium concentration depends on the concentration of this micronutrient in food, which in turn depends on selenium content in soil, bioavailability of selenium, which is different in various geographical regions, and forms of selenium. Selenium circulating in blood as a component of selenoproteins participates in oxidoreduction, thus reducing the risk of developing colorectal cancer. Despite this well-known protective mechanism against colorectal cancer occurrence, half of the reviewed studies did not confirm the protective properties of selenium. To sum up, the current state of knowledge on the association between selenium and colorectal cancer or adenoma has revealed not only inconclusive results of the studies, but has also shown that there is a need to conduct more prospective studies.