ABSTRACT: Interactions between phytochemical components often modify the pharmacological effects of botanical dietary supplements, functional foods, or drugs. These interactions can either potentiate the effect of bioactive phytochemicals or interfere with their activity. This review defines and explores 2 types of phytochemical interactions: endointeractions that occur between components within a plant species and exointeractions that occur between components from different plants or between plants and synthetic drugs. Exointeractions and endointeractions between and within the complex mixtures of phytochemicals may have a profound effect on human health. Few well-characterized interactions were shown to affect a wide range of biological processes such as metabolism, bioavailability, solubility, cellular uptake and efflux, and body clearance. Phytochemical interactions may explain the health effects of regional diets, undesirable side effects of drugs, and inconsistent performance of dietary supplements. Better understanding of health-related phytochemical interactions should lead to a more sophisticated, holistic approach to disease prevention and treatment.