The interplay between ATM and DNA-PKcs kinases during double strand breaks (DSBs) resolution is still a matter of debate. ATM and DNA-PKcs participate differently in the DNA damage response pathway (DDR), but important common aspects are indeed found: both of them are activated when faced with DSBs, they share common targets in the DDR and the absence of either kinase results in faulty DSB repair. Absence of ATM translates into timely repair that, nevertheless, is incomplete. On the other hand, DNA-PKcs absence translates into slower repair, which in turn gives rise to the accumulation of simple and complex reorganizations. These outcomes confirm that the function of both protein kinases is essential to guarantee genome integrity. Interestingly, V(D)J and CSR recombination events provide a powerful tool to study the interplay between both kinases in DSB repair. Although the physiological DSBs generated during V(D)J and CSR recombination are resolved by the non-homologous end-joining (NHEJ) repair pathway, ATMabsence during these events translates into chromosome translocations. These results suggest that NHEJ accuracy is threatened in the absence of ATM, which may play a role in avoiding illegitimate repair by favouring the joining of the correct DNA ends. Indeed, simultaneous DNA-PKcs and ATM deficiency during V(D)J and CSR recombination translates into a synergistic increase in potentially dangerous chromosomal translocations and deletions. Although the exact nature of their interaction remains elusive, the evidence indicates that ATM and DNA-PKcs play complementary roles that allow complete and legitimate DSB repair to be reached. Faithful repair can only be achieved by the presence and correct functioning of both kinases: while DNA-PKcs ensures fast rejoining, ATM guarantees complete repair.