Members of the phosphatidylinositol 3' kinase (PI3K)-related mutated (ATM), ataxia-telangiectasia mutated and Rad3related (ATR), mammalian target of rapamycin (mTOR), suppressor with morphological effect on genitalia 1 (SMG1), and transformation/transcription domain-associated protein 1 (TRRAP/Tra1), participate in a variety of physiological processes, such as cell-cycle control, metabolism, transcription, replication, and the DNA damage response. In eukaryotic cells, DNA-PKcs, ATM, and ATR-ATRIP are the main sensors and regulators of DNA double-strand break repair. The purpose of this review is to describe recent structures of DNAPKcs, ATM, and ATR, as well as their functions in activation and phosphorylation in different DNA repair pathways.