Probiotics are defined as live microorganisms that, when administered in adequate amounts, confer a health benefit to the host beyond their nutritional contribution. In addition to their effects on gut homeostasis, probiotics may also regulate systemic immune and inflammatory responses. Therefore, modulating the gut microbiota by orally administered probiotics could exert positive effects on the skin. The cutaneous disease in which these effects have been most widely studied is atopic dermatitis, and greater benefits have been obtained in the prevention than in the treatment of this entity. Prebiotics, in contrast, are non-digestible food ingredients that beneficially affect the host by selectively stimulating bacterial growth and/or activity. This concept was originally developed to selectively modify the gut microflora, but in principle could be applied to modulate the composition of any microbial community, including the skin microflora. A prominent example is the skin of acne patients, in which prebiotic administration could be effective in reducing the amount of Propionibacterium acnes, and at the same time in preserving or even stimulating the growth of beneficial bacteria. Continued research is required before solid, evidence-based recommendations can be made. © 2010 Elsevier España, S.L. Todos los derechos reservados.