Group 1 innate lymphoid cells (ILCs) includes both conventional natural killer cells (NK cells) and ILC1s, which are two cell types that are involved in host defense against intracellular viral and bacterial pathogens and protozoan parasites. NK cells and ILC1s are classified together as group 1 ILCs as they share a number of common features including that they both secrete IFN-gamma following activation, and require the transcription factor, T-bet, for their development and function. Additionally, both NK cells and ILC1s primarily require IL-15 rather than IL-7 for their development, they are both activated by IL-12, IL-15, and IL-18, and express some of the same cell surface markers including NKp46, CD122/IL-12 R beta, and NK1.1 in mouse. Unlike ILC1s, however, NK cells are derived from a different progenitor, lack expression of CD127/IL-7 R alpha, and express the transcription factor, Eomes in addition to T-bet, as well as receptors belonging to the mouse Ly49 or human killer immunoglobulin-like receptor (KIR) families. Furthermore, NK cells also express granzymes and Perforin and have cytotoxic activity, while ILC1s have only weak cytotoxic potential. As ILCs are classified based on their similarities with different effector T cell subsets, NK cells are considered to be the innate counterpart of CD8+ T cells, while ILC1s are the innate equivalent of Th1 cells. The unique functions of NK cells and ILC1s in tumor rejection and viral infection are currently being investigated.