Innate lymphoid cells (ILCs) are newly described immune lymphoid cells that are morphologically similar to B cells and T cells but lack rearranged antigen receptors. ILCs secrete high concentrations of cytokines and are implicated in innate immunity, inflammation, lymphoid tissue formation, and tissue remodeling. Consistent with their role in immune surveillance, and their implication in early detection of pathogens, ILCs are localized to mucosal surfaces and respond to secreted molecules from the epithelium. All ILC populations differentiate from a common lymphoid progenitor (CLP), which is localized to the fetal liver or adult bone marrow, in response to the expression of specific transcription factors. Because ILCs share developmental and functional similarities with helper T (Th) cells, nomenclature for ILCs has been established based on Th cells classification. ILCs are categorized into three groups according to the transcription factors mediating their development and the cytokines they secrete. Group-1 ILCs are under the control of the T-bet transcription factor and include natural killer (NK) cells and ILC1 cells. They secrete type-1 cytokines such as IFN-gamma and TNF-alpha in response to intracellular pathogens. Group-2 ILCs rely on the GATA-3 and ROR-alpha transcription factors and produce type-2 cytokines (IL-5, IL-9, IL-13) in response to extracellular parasite infections. Finally, Group-3 ILCs composed by Lymphoid Tissues inducer cells (LTi) and ILC3 cells, is under the control of the ROR-gamma t transcription factor and includes cells that produce IL-17 and/or IL-22. LTi cells are required for the development of lymphoid tissues, while ILC3 cells mediate the balance between intestinal symbiotic microbiota and immunity.