Innate lymphoid cells (ILCs) are important effectors and regulators involved in immune surveillance and in the preservation of tissue integrity. Consistent with their implication in early detection of pathogens, ILCs are present at mucosal surfaces and respond to secreted molecules from the epithelium. They are derived from a common lymphoid progenitor cell (CLP) that originates from fetal liver or adult bone marrow. Sequentially expressed transcription factors orchestrate the differentiation of the CLP into either an LTi progenitor cell (LTiP), an NK progenitor cell (NKP), or an ILC progenitor (ILCP). Because ILCs share developmental and functional similarities with helper T (Th) cells, a nomenclature for ILCs has been established based on Th cell classification. ILCs are categorized into three groups according to the transcription factors mediating their development and the cytokines they secrete. Predominantly localized in the intestinal lamina propria (LTi cells, NCR-ILC3, NK cells and ILC1) and epithelium (Intraepithelial ILC1), group-1 and 3 ILCs mediate the balance between symbiotic microbiota and intestinal immune system and are implicated in the response against pathogenic gut bacteria. Group-2 ILCs have been described as part of fat-associated lymphoid clusters in human and mouse mesentery, and are present in the airway mucosa and the skin of healthy human and mouse.