Apoptosis, or physiological cell death, is the way in which unwanted cells are removed. The majority of cells formed during haemopoiesis are destined to die by apoptosis before they are fully differentiated, and homeostasis of cell number is maintained by a balance between mitosis and apoptosis. Many haematological malignancies are associated with changes in the number of cells undergoing apoptosis, which may be a direct or an indirect effect. Genetic mutations that prevent cell death cause cells to accumulate and can eventually lead to malignancy. Alternatively, oncogenic mutations that lead to increased cell production can indirectly cause a decrease in apoptosis in some populations and an increase in others. Chemotherapeutic drugs may kill cells directly, or indirectly by inducing apoptosis as a stress response. Therapeutic strategies are evolving to increase the propensity of malignant cells to die by either means and to mitigate side effects by reducing apoptosis in non-malignant cells.