Group 3 innate lymphoid cells (ILC3s) are a heterogeneous population of cells that like other ILC subsets, are primarily located at mucosal sites such as the lung and intestine. Two subsets of ILC3s have been described in mouse and human: natural cytotoxicity receptor (NCR)+ ILC3s and (NCR)- ILC3s, which includes fetal lymphoid tissue inducer (LTi) cells and adult LTi-like ILC3s. Both (NCR)+ and (NCR)- ILC3s are thought to be involved in initiating the immune response against extracellular microbial pathogens and maintaining tissue homeostasis in postnatal organisms, while fetal LTi cells are required for the development of secondary lymphoid organs during embryogenesis. All ILC3s require the transcription factor, ROR gamma t, for their development and produce IL-22 and/or IL-17. As a result, (NCR)+ and (NCR)- ILC3s have been suggested to be the innate counterparts of Th22 and Th17 cells, respectively.