Summary BCL6 is a transcription factor that has essential B‐cell and T‐cell roles in normal antibody responses. It is involved in chromosomal translocations in diffuse large B‐cell lymphoma (DBCL; including primary mediastinal B‐cell lymphoma) and nodular lymphocyte predominant Hodgkin lymphoma, and is expressed in follicular lymphoma and Burkitt's lymphoma. The neoplastic T‐cells of angioimmunoblastic T‐cell lymphoma also express BCL6. BCL6 prevents terminal B‐cell differentiation largely through repression of PRDM1. In the “cell of origin” classification of DLBCL BCL6 is associated with the germinal centre subtype, which carries a good response to modern treatments. More recently, specific BCL6 antagonists, including small molecule inhibitors, have been developed. These antagonists have demonstrated that DLBCL cells, in which BCL6 is transcriptionally active, are dependent on this gene for survival. BCL6 antagonists are active against primary DLBCL and may find future application in the treatment of lymphomas.