Natural Killer (NK) cells were called as such due to their ability to spontaneously kill tumor cell lines in vitro. Peripheral NK cells have long been considered rapid responding innate effector lymphocyte primed to produce cytotoxic granules and pro-inflammatory cytokines such as IFN-γ and TNF-α when they encounter virusinfected, transformed or damaged self-tissue. However, it is becoming clear that considerable heterogeneity exists with the peripheral NK cell pool and researchers should reconsider the generalized view of NK cell effector function and contribution to mammalian immunity. This is extremely relevant to tissues such as the liver, salivary gland and intestine where type 1 innate lymphoid cells (ILC1) are found and resemble NK cells in many regards yet have distinct transcription factor requirements to conventional NK cells.