Carotenoids, one of the most studied groups of molecules, still represent a great enigma for the world of science regarding their antioxidant or prooxidant activity. Although the mechanisms of antioxidant activity have been studied in detail, carotenoids are surprising with their prooxidant activity under certain conditions, both in vitro and in vivo. The question for us is, what conditions lead to the apparently strange and unexpected behavior of the carotenoids? To answer these questions, we need to submerge deeper into the chemical and physical interactions of carotenoids with reactive species, primarily oxygen reactive species, and singlet oxygen in the first place. Do the established mechanisms provide answers to these questions or should we reconsider the possibility that side reactions of the chemical interaction of carotenoids with reactive species have a non-negligible role in their behavior? These are questions that need to continue to be asked. If we clear them out, we will enable the safer use of carotenoids as dietary supplements, but also their safer use in pharmacy, cosmetics, and other branches of industry where they have been used for a long time.