The immunomodulatory properties of lactic acid bacteria (LAB) and foods containing them (e.g., fermented milks) is a topic currently under investigation. Individuals could potentially benefit from the inclusion of LAB in the diet at different times during the life cycle. One of the most accepted specific uses of probiotic bacteria is the prevention of atopic eczema in infants with family history of the disease who receive the probiotic bacteria early, through supplementation of the gestating mother and orally after birth. Immune enhancing effects have also been suggested to be beneficial in diarrhoea treatment, especially in children infected with rotavirus and in malnourished patients, infants and adolescents, whose capacity to produce IFN-gamma can be increased after LAB-containing yoghurt intake. Regarding young people and adults, investigations have been conducted exploring the immunomodulation by LAB in subjects under stressful situations, in the prevention of urinary tract infections in fertile women and in the treatment of allergy. However, the beneficial effects of probiotics in these conditions remain controversial and the scientific evidence provided so far is not considered to be conclusive. The elderly population has been the focus of investigations aimed at identifying the capacity of probiotics to counteract the immunosenescence process by increasing phagocytic and natural killer (NK) cell activities and to protect against infection. The mechanisms involved in the different effects attributed to LAB remain to be clarified. Moreover, considering that the immunomodulatory properties are strain-specific, defining the optimal dose of a certain bacteria or combination of bacteria strains and the duration of treatment for a desired effect in a target population group is essential in order to substantiate health claims.