ILC3s, from embryo to adult.Group 3 innate lymphoid cells (ILC3) represent a heterogeneous population of cells that share the nuclear hormone receptor ROR gamma t (retinoic acid receptor-related orphan receptor gamma t) as a master regulator for differentiation and function. ILC3 can be divided into two major subsets based on the cell surface expression of the natural cytotoxicity receptor (NCR), NKp46. A subset of NCR- ILC3 includes the previously known lymphoid-tissue inducer cells that are essential for the embryonic formation of peripheral lymph nodes and Peyer's patches. After birth, the NCR- and NCR+ ILC3 contribute to the maintenance of health but also to inflammation in mucosal tissues. This review will describe the differentiation pathways of ILC3, their involvement in the development of the adaptive immune system and their role in the establishment and maintenance of gut immunity.