JunB is a member of the AP-1 (activator protein-1) family of dimeric transcription factors. It exerts a dual action on the cell cycle. It is best known as a cell proliferation inhibitor, a senescence inducer and a tumour suppressor. As for the molecular mechanisms involved, they largely involve both positive actions on genes such as the p16INK4alpha cyclin-dependent kinase inhibitor and negative effects on genes such as cyclin D1 during the G1-phase of the cell cycle. However, JunB is also endowed with a cell-division-promoting activity, in particular via stimulation of cyclin A2 gene expression during S-phase. Strikingly, its role in G2 and M has received little attention so far despite its possible role in the preparation of mitosis. This review addresses the known and possible mechanisms whereby JunB is implicated in the control of the different phases of the cell cycle.