The growing interest in functional foods during the past few years has led to an increasing number of papers in the literature reporting on biological effects of various probiotic bacterial strains, prebiotics and functional carbohydrates, antioxidants and other dietary modulators of metabolic functions. In this paper, we will present the criteria of selection of bioactive microorganisms and molecules occurring - or to be incorporated - in functional foods, the scientific basis of their health benefits and the future requirements for new product developments.