Probiotics, a word of Greek origin composed of "pro" (promoting) and "biotic "(life), have been defined by the Food and Agriculture Organization of the United Nations (FAO) as "live microorganisms, which, when administered in adequate amounts, confer a health benefit on the host". Over 500 different types of bacteria reside in the human digestive system. In recent years, the study of intestinal microbiota and therefore the use of probiotics have been extensively studied in promoting health, as well as in treating several diseases and not solely regarding the gastrointestinal system. This book summarises the more recent knowledge about intestinal microbiota, its pathological correlations and the use of probiotics in children for the maintenance, recovery, and strengthening of healthy strains at the expense of bad ones.