The intestinal flora consists of a considerable mass of 10(14) cells which accompany the individual step by step from birth to death. It builds up multiple relations with the host housing it and represents a complex ecosystem, in dynamic equilibrium. The stability of the latter is maintained by virtue of only partially understood interactions. Gastric acidity, peristalsis and the immune system participate in this stability. However, this ecosystem is constantly subject to the actions of the outside medium, whether in the form of exogenous bacteria which are ingested on a constant basis of dietary substances. This fight against infection is one of the key factors in the role of the ecosystem. The actions of the flora are nevertheless far more extensive. It may also act on the host and, taking all populations into account, it is easy to see the enormous enzyme potential and the possibility of a range of metabolic effects, nutritional in particular. However, the state of equilibrium is fragile. Disturbances in the flora secondary to antibiotics and to bacterial infections cause disorders which are transitory as a rule. Equilibrium is most often restored by endogenous means (mucus, immune system, peristalsis).