DNA double-strand breaks are associated with various endogenous processes, such as transcription, recombination, replication, and with the process of active cell death, which aims to eliminate cells. In addition, DNA double-strand breaks can be induced by irradiation, exposure to chemicals, increased formation of reactive oxygen species, and, indirectly, during repair of other types of DNA damage or as a consequence of extranuclear lesions. In addition to the neutral filter elution of DNA, the recently introduced pulsed-field gel electrophoresis is capable of determining DNA double-strand breaks with higher accuracy and sensitivity and is expected to increase our knowledge on the frequency and the role of DNA breakage. Parallel determination of parameters for cytotoxicity is necessary to elucidate the causal primary lesion. Although the repair of DNA double-strand breaks is a complex task, cells are capable of repairing--with or without errors and up to a certain extent--and surviving this DNA lesion. Gene translocations, rearrangements, amplifications, and deletions arising during repair and misrepair of double-strand breaks may contribute to cell transformation and tumor development.