Humans live in a mutual beneficial relationship with the commensal microbiota that colonize the epithelial surfaces of the body. The remarkable impact of commensal microbiota on the mucosal and systemic immune system has become apparent during the last years. It is now believed that changes in the composition of the intestinal microbiota as a consequence of “Western” lifestyle, significantly contribute to the rising incidence of autoimmune and allergic diseases. There is evidence that exposure to certain microbes during early childhood or even to the maternal microflora in utero are important factors in shaping the health of the neonate/infant.The objective of this project is to understand the molecular mechanism into how signals originating from maternal microbiota can shape fetal and neonatal immunity. Using a model of transient colonization of pregnant mice kept under germ-free conditions, we have discovered a beneficial effect of maternal microbiota on the immune system of the offspring. Importantly, mice born from transiently colonized mothers exhibited higher numbers of innate lymphoid cell populations in the intestine compared to offspring from untreated germ-free mothers. We suggest two approaches to depict how and by which route the maternal microbiota calibrates the immune system of the offspring.Besides increased appearance of allergic diseases in children in developed countries, neonate and child mortality from infectious diseases are still very high in developing countries. Thus, the improvement of neonatal immunity is an important goal. Our research will contribute to the development of new therapeutic tools to reduce child mortality as well as for the prenatal prevention of chronic inflammatory and allergic diseases.