Innate lymphoid cells (ILCs) are defined as lymphocytes that lack RAG recombinase and do not express diverse antigen receptors;however,recent studies have revealed the adaptive features of ILCs.Mouse cytomegalovirus (MCMV)-and cytokine-induced memory natural killer (NK) cells circulate in the blood and are referred to as conventional memory NK cells.In contrast,virusand hapten-induced memory NK cells,hapten-induced memory ILC1s,and cytokine-induced memory-like ILC2s exhibit long-term residency in the liver or lung,and are referred to as tissue-resident memory ILCs.Considering their similar migration pattems and memory potential,tissue-resident memory ILCs could be regarded as innate counterparts of resident memory T (TRM) cells.Both tissue-resident memory ILCs and TRM cells share common characteristics in terms of dynamics,phenotype,and molecular regulation.The emergence of ILC memory expands the basic biology of ILCe and prompts us to re-examine their functions in disease progression.This review discusses the evidence supporting tissue-resident memory NK cells and other memory ILC subsets,compares them with TRM cells,and highlights key unsolved questions in this emerging field.