BCL6 is a transcriptional repressor required in mature B cells during the germinal center (GC) reaction. Multiple mechanisms act coordinately to timely modulate BCL6 expression at transcriptional and post‐transcriptional levels. BCL6 prevents premature activation and differentiation of GC B cells and provides an environment tolerant of the DNA breaks associated with immunoglobulin gene remodeling mechanisms involved in the production of high‐affinity antibodies of different isotypes. The critical functions exerted by BCL6 during normal B‐cell development can be hijacked by the malignant transformation process. Indeed, BCL6 is targeted by genetic aberrations and acts as an oncogene in GC‐derived lymphomas. The aberrations affecting BCL6 interfere with the multiple levels of regulation that grant a fine tuning of BCL6 expression and activity in physiologic conditions. This review summarizes the current knowledge on BCL6 function and its role in lymphomagenesis.