Epidemiological evidence from the last fifty years has demonstrated that nutrition plays a decisive role in human health. Eating properly is not only necessary to meet energy demands. It also actively contributes, through both preventive actions and therapeutic effects, to improving human wellness. Nutrition owes its functional role in human health to the biological activity of specific, small dietary molecules. Plants are the most important source of bioactive molecules, and dietary phytochemicals are mainly responsible for the documented protective effects of diets which are rich in plant foods. Dietary phytochemicals have attracted increasing interest in human nutrition research over the past few years due to their ability to exert several biological effects that are potentially useful for human health, In this Special Issue, the biological activity of dietary phytochemicals, either purified or in extracts from plant foods, and their potential effects on human health are addressed and investigated.