The nonhomologous DNA end joining (NHEJ) pathway is a critical mechanism for repairing double-strand DNA breaks (DSBs) in eukaryotic cells. It is distinct from homology-directed repair (HDR) and is essential for repairing both pathologic and physiologic DSBs, such as those generated during V(D)J recombination and class switch recombination. NHEJ is highly flexible and can act on a wide range of DNA end configurations, including those with oxidative damage or varied overhangs. This flexibility allows NHEJ to function independently at each DNA end being joined, making it a key player in DNA repair across diverse cellular contexts. NHEJ involves a complex network of proteins, including Ku, DNA-PKcs, Artemis, XRCC4, and DNA ligase IV. Ku is the first protein to bind to DNA ends, facilitating the recruitment of other NHEJ enzymes. Artemis and DNA-PKcs have diverse nuclease activities that help process DNA ends, while XRCC4 and DNA ligase IV are crucial for ligation. The flexibility of these enzymes allows NHEJ to handle a wide variety of DNA end structures, contributing to the diversity of repair outcomes. NHEJ is particularly important in the absence of homology donors, such as in nondividing cells or during S/G2 phases. It is also essential for repairing DSBs caused by ionizing radiation, reactive oxygen species, and enzymatic errors. The presence of terminal microhomology between DNA ends can significantly enhance the efficiency of NHEJ, reducing the need for extensive resection. However, microhomology is not essential for NHEJ, as alternative mechanisms can also facilitate repair. In vertebrates, NHEJ is highly mechanistically flexible, with enzymes that can act in any order and independently at each DNA end. This flexibility is crucial for repairing DSBs in the absence of homology donors, such as those encountered in lymphoid cells during antigen receptor gene rearrangement. The NHEJ pathway is also important in meiotic cells, where it is not typically involved in repairing DSBs generated by the Spo11 enzyme, which are resolved by HDR. The NHEJ pathway is essential for maintaining genomic stability and preventing immunodeficiency in individuals with defective NHEJ. Mutations in NHEJ components can lead to severe immunodeficiency and increased sensitivity to ionizing radiation. Understanding the mechanisms of NHEJ is crucial for developing therapeutic strategies for diseases associated with DNA repair defects.The nonhomologous DNA end joining (NHEJ) pathway is a critical mechanism for repairing double-strand DNA breaks (DSBs) in eukaryotic cells. It is distinct from homology-directed repair (HDR) and is essential for repairing both pathologic and physiologic DSBs, such as those generated during V(D)J recombination and class switch recombination. NHEJ is highly flexible and can act on a wide range of DNA end configurations, including those with oxidative damage or varied overhangs. This flexibility allows NHEJ to function independently at each DNA end being joined, making it a key player in DNA repair across diverse cellular contexts. NHEJ involves a complex network of proteins, including Ku, DNA-PKcs, Artemis, XRCC4, and DNA ligase IV. Ku is the first protein to bind to DNA ends, facilitating the recruitment of other NHEJ enzymes. Artemis and DNA-PKcs have diverse nuclease activities that help process DNA ends, while XRCC4 and DNA ligase IV are crucial for ligation. The flexibility of these enzymes allows NHEJ to handle a wide variety of DNA end structures, contributing to the diversity of repair outcomes. NHEJ is particularly important in the absence of homology donors, such as in nondividing cells or during S/G2 phases. It is also essential for repairing DSBs caused by ionizing radiation, reactive oxygen species, and enzymatic errors. The presence of terminal microhomology between DNA ends can significantly enhance the efficiency of NHEJ, reducing the need for extensive resection. However, microhomology is not essential for NHEJ, as alternative mechanisms can also facilitate repair. In vertebrates, NHEJ is highly mechanistically flexible, with enzymes that can act in any order and independently at each DNA end. This flexibility is crucial for repairing DSBs in the absence of homology donors, such as those encountered in lymphoid cells during antigen receptor gene rearrangement. The NHEJ pathway is also important in meiotic cells, where it is not typically involved in repairing DSBs generated by the Spo11 enzyme, which are resolved by HDR. The NHEJ pathway is essential for maintaining genomic stability and preventing immunodeficiency in individuals with defective NHEJ. Mutations in NHEJ components can lead to severe immunodeficiency and increased sensitivity to ionizing radiation. Understanding the mechanisms of NHEJ is crucial for developing therapeutic strategies for diseases associated with DNA repair defects.