Programmed cell death, also commonly referred to as apoptosis, is a genetically controlled sequence of events, often requiring protein synthesis, which results in cell death. Although initially described 20 years ago, it is only recently that its role as a mechanism in controlling cell population size and content has been fully realised. In this review the biochemical events of apoptosis are discussed briefly, followed by a more detailed look at the role of programmed cell death in the control of haemopoiesis and the maintenance of a balanced functional immune system. The role of proto-oncogenes and suppressor genes known to regulate programmed cell death is covered, and also their influence on the induction and maintenance of neoplastic disease. Finally, the potential role of apoptosis in the resistance of haematological malignancies to chemotherapy is commented on.