Overexpression of members of the BCL-2 family of pro-survival proteins is commonly associated with unfavourable pathogenesis in cancer. The convergence of cytotoxic stress signals on the extended BCL-2 protein family provides the biological rationale for directly targeting this family to induce apoptotic cell death. Recently, several compounds have been described that inhibit the interaction between BCL-2 family members and their natural ligand, a helical peptide sequence known as the BH3 domain. Here, we review preclinical and clinical data on these compounds, and recommend four criteria that define antagonists of the BCL-2 protein family.