Because cancer at its origin must acquire permanent genomic mutations, it is by definition a disease of DNA repair. Yet for cancer cells to replicate their DNA and divide, which is the fundamental phenotype of cancer, multiple DNA repair pathways are required. This produces a paradox for the cancer cell, where its origin is at the same time its weakness. To overcome this difficulty, a cancer cell often becomes addicted to DNA repair pathways other than the one that led to its initial mutability. The best example of this is in breast or ovarian cancers with mutated BRCA1 or 2, essential components of a repair pathway for repairing DNA double-strand breaks. Because replicating DNA requires repair of DNA double-strand breaks, these cancers have become reliant on another DNA repair component, PARP1, for replication fork progression. The inhibition of PARP1 in these cells results in catastrophic double-strand breaks during replication, and ultimately cell death. The exploitation of the addiction of cancer cells to a DNA repair pathway is based on synthetic lethality and has wide applicability to the treatment of many types of malignancies, including those of hematologic origin. There is a large number of novel compounds in clinical trials that use this mechanism for their antineoplastic activity, making synthetic lethality one of the most important new concepts in recent drug development.