The phospho-Ser/Thr-Pro specific prolyl-isomerase Pin1 has been implicated in multiple aspects of cell cycle regulation. It has been suggested that Pin1 function is required for both normal mitotic progression and reentry into the cell cycle from quiescence. In support of this hypothesis, numerous key regulators of G1 and mitosis have been identified as Pin1 interacting proteins. However, the cellular consequence of Pin1 binding to these proteins has rarely been rigorously characterized. In this review we focus on the role of Pin1 and its binding proteins in cell cycle regulation and the potential value of Pin1 as a therapeutic target.