Now considered an organ with defensive and metabolic capabilities, the intestinal microbiota plays a major role in the local host immune system development and education. It contributes to the generation of a homeostatic balance characterized by the capacity to react against pathogens while remaining hyperresponsive/tolerant against commensals. This homeostatic response depends on bacteria and bacterial product sensing by innate immune cells and their molecular asset at the intestinal mucosa. This book captures the enormous progress that has been accomplished in this field in recent years.