Antitumor activities of group 1 innate lymphoid cells (ILC1). Upon tumor development dendritic cells (DC) are activated and secrete IL-12, which activates ILC1. ILC1 respond to stimulation secreting IFN-γ and TNF-α, which target and activate different cell types within the tumor microenvironment that display antitumor activities. These cell types include NK cells that kill tumor cells through mechanisms involving perforin and granzyme secretion. CD4+ T cells provide costimulation (through cell to cell interactions and secretion of soluble factors) and priming of CD8+ cytotoxic T cells which display antitumor cytotoxic activities. IFN-γ secreted by ILC1 inhibits the differentiation of tumor-infiltrating macrophages into M2 macrophages providing a mechanism that prevents secretion of vascular endothelial growth factor (VGEF) and tumor angiogenesis. Due to cell plasticity ILC3 can differentiate into ILC within the tumor microenvironment and contribute to the anti- and protumor responses.