Hormesis is represented by an adaptive response, in which exposure to low or moderate doses increases the defensive capacity that is collapsed under high-dose conditions. In recent decades, there is emerging solid evidence that food factors, particularly phytochemicals, exert diverse biofunctions partially through the hormesis-associated mechanisms of action. These phenomena are derived from the fact that phytochemicals, which are biosynthesized as the secondary metabolites in plants, are substantially xenobiotics to animals. Previous reports have revealed that many polyphenols induce oxidative stress via autoxidation under certain biological conditions. Similarly, some electrophilic phytochemicals may induce proteostress by reacting with a thiol moiety of the cysteine residue of cellular proteins. In addition, phytochemicals are well described to upregulate the expression of xenobiotics-metabolizing enzymes for their excretion. Repetitive adaptation responses may potentiate the defensive capacity, and this phenomenon can be described as “chemical training.” Moreover, such hormetic responses may be partially involved in the mechanisms underlying the already-known biofunctions of phytochemicals. This chapter highlighted the emerging importance of hormesis in the research field of functional food factors.