The concept of functional food, about which scientific agreement is still lacking, springs from the field of Optimum Nutrition, aimed at modifying genetic and physiological aspects of human life and at the prevention and treatment of a growing number of diseases, far beyond merely covering nutritional requirements. From the European Union perspective, functional foods can be natural as well as industrially processed foods. The leading functional foods regarding which the soundest scientific evidence exists are probiotics, live microbial food ingredients represented mainly by fermented dairy products. Prebiotics, such as inulin-type fructans, are the trophic substrate of probiotics and potential intestinal microflora selectors. The combination of prebiotics and probiotics is termed synbiotic. Innumerable substances are known to have functional effects: soluble and insoluble fiber, phytosterols, phytoestrogens, monounsaturated and polyunsaturated fatty acids, phenol derivatives, vitamins and other phytochemicals. Functional foods exert their actions on different systems, especially the gastrointestinal, cardiovascular and immunological ones, acting too as enhancers of development and differentiation and positively modulating nutrient metabolism, gene expression, oxidative stress and the psychic sphere. The establishment of Health Claims must be firmly based upon scientific knowledge and legal regulation. Efficient biomarkers related to biological response must be found. Furthermore, it is essential to analyze possible diet or drug interactions as well as it is indispensable to conduct valid studies on humans. The prime objective must be the diet as a whole. Thus, the future challenge of a functional diet emerges.