Natural killer (NK) cells are the founding member of the innate lymphoid cell (ILC) family. They are widely distributed all around the body, consisting of phenotypically and functionally distinct subsets, and the composition of NK cell subsets varies with tissue distribution. In the liver, NK cells are particularly enriched and contain a relatively large fraction of liver-resident NK (LrNK) cells, also categorized as type 1 ILCs (ILC1s). This cell subset is distinct from conventional NK (cNK) cells, which are circulating and predominantly present in various tissues. Herein, this chapter introduces the identification and developmental origin of LrNK cells/ILC1s, presents an overview of recent advances in the transcriptional factors specifically or commonly required by LrNK cells/ILC1s and cNK cells, and highlights division of labor between liver NK cell subsets during viral infection and liver cancer.