Probiotics are non-pathogenic microorganisms that are ingested live in order to cause beneficial effects on the body. These beneficial effects include increased resistance to infection, reduction of hypercholesterolemia, attenuation or reduction of allergies, reduction of tumourigenesis, inflammatory activity and others. This line is concerned with the selection of new probiotic strains of bacteria, especially species of the genus Lactobacillus, their characterisation as probiotics, the study of their mechanisms of action and their possible uses in nutritional supplements such as fermented milk derivatives.