The population is aging. Over the coming years, the incidence of age-related chronic diseases such as cancer is expected to continue to increase. Phytochemicals, which are non-nutritive chemicals found in plants and food, have emerged as modulators of key cellular signaling pathways exerting proven anticancer effects. The challenge now is to develop personalized supplements comprised of specific phytochemicals for each clinical situation. This will be possible once a better understanding is gained of the molecular basis explaining the impact of phytochemicals on human health. The aim of the present literature review is to summarize current knowledge of the dietary phytochemicals with proven antitumor activity, with a special emphasis placed on their molecular targets. Also discussed are the limits of existing research strategies and the future directions of this field.