CD27 is a co-stimulatory immune-checkpoint receptor, constitutively expressed on a broad range of T-cells (αβ and γδ), NK-cells and B-cells. Ligation of CD27 with CD70 results in potent co-stimulatory effects. In mice, co-stimulation of CD8+ T-cells through CD27 promotes immune activation and enhances primary, secondary, memory and recall responses towards viral infections. Limited in vitro human studies support mouse experiments and show that CD27 co-stimulation enhances antiviral T-cell immunity. Given the potent co-stimulatory effects of CD27, manipulating CD27 signalling is of interest for viral, autoimmune and anti-tumour immunotherapies. This review focuses on the role of CD27 co-stimulation in anti-viral T-cell immunity and discusses clinical studies utilising the CD27 co-stimulation pathway for anti-viral, anti-tumour and autoimmune immunotherapy.