The last few years has seen the burgeoning of a new category of therapeutics for cancer targeting immune regulatory pathways. Antibodies that block the PD-1/PD-L1 interaction are perhaps the most prominent of these new anti-cancer therapies, but several other inhibitory receptor ligand interactions have also shown promise as targets in clinical trials, including CTLA-4/CD80 and Lag-3/MHC class II. Related to this is a rapidly improving knowledge of 'regulatory' lymphocyte lineages, including NKT cells, MATT cells, B regulatory cells and others. These cells have potent cytokine responses that can influence the functioning of other immune cells and many researchers believe that they could be effective targets for therapies designed to enhance immune responses to cancer. This review will outline our current understanding of FOXP3 + 'Tregs', NKT cells, MAIT cells and B regulatory cells immune regulatory cell populations in cancer, with a particular focus on chronic lymphocytic leukaemia (CLL). We will discuss evidence linking CLL with immune regulatory dysfunction and the potential for new therapies targeting regulatory cells. (C) 2015 Elsevier B.V. All rights reserved.