Summary The B‐cell lymphoma 2 (BCL2) family of proteins comprise key regulators of apoptosis and are implicated in the pathogenesis of many malignancies, including lymphomas and leukaemias. Targeting of BCL2 proteins can be directly toxic to tumour cells or render them more sensitive to chemotherapy. Inhibition of the anti‐apoptotic functions of BCL2 proteins using structure‐based design to produce specific inhibitors of protein–protein interactions has been achieved for BCL2, MCL1 and BCL‐XL (also termed BCL2L1), providing an armamentarium of new targeted therapies called BH3‐mimetics. The first BCL2‐specific inhibitor, venetoclax, has shown extraordinary single agent activity in chronic lymphocytic leukaemia (CLL), with surprisingly little toxicity given the expression of BCL2 in normal tissues. Despite success in CLL, where sensitivity to BCL2 inhibition is seen in nearly all cases, key questions have not yet been addressed. For example, responses to venetoclax in other B‐cell and myeloid malignancies are heterogeneous, highlighting the need to identify biomarkers that correlate with response and, secondly, to identify/develop other specific compounds that synergise with BCL2 inhibition. In this review, we summarise the biology of BCL2 proteins, the mechanism of action of BH3‐mimetics and the status of their clinical development in haematological malignancies.