Several traditional nutrients and dietary fiber have been associated with both increased and decreased risk of chronic diseases. However, there are many minor components in foods, particularly plant-derived foods, that elicit biologic responses in mammalian systems that are consistent with reduced risk of one or more chronic diseases (phytonutrients). These phytonutrients have been categorized into ten classes of compounds or biologic activities. Representative compounds, typical biologic activities, and common food sources are tabulated for each phytonutrient class. A brief discussion of each category is presented along with several structure-activity relationships.