Summary of pathways and signals discussed in this review that induce NK cell activation and/or NKG2D ligand expression in tumor cells and in virus-infected cells. The pathways include the DNA damage response, the cGAS-STING pathway, proliferative signals, PI3-kinase activity, dysregulation of histone acetylation, and the integrated stress response. Most of these pathways to NK cell activation are deployed in both cancer cells and in viral infections, as indicated in the figure and in Table 2.