Mitotic entry requires the activation of cyclin B–Cdk1 and Greatwall (Gwl; also known as MASTL in mammals) kinases. Together, these kinases phosphorylate mitotic substrates and ensure that they remain phosphorylated by repressing the activity of PP2A-B55, the phosphatase responsible for substrate dephosphorylation. For cells to exit mitosis, cyclin-B–Cdk1 and Gwl/MASTL kinases are inactivated, and phosphatase activity is restored. But what deactivates Gwl/MASTL, and how is this is achieved? In this issue, two groups address this question and identify protein phosphatase-1 (PP1) as the phosphatase that triggers the dephosphorylation of Gwl/MASTL. On page 1329, Anna Castro, Thierry Lorca and colleagues show that depletion of PP1 from meiotic Xenopus egg extracts promotes the stabilisation of Gwl on Ser875, one of the key activating sites controlling Gwl activity, and prevents exit from meiosis and mitosis. By contrast, preventing PP2A-B55 reactivation following meiotic and mitotic exit substantially delays Gwl phosphorylation on Thr194, the second essential activating site on the kinase. Moreover, the authors show that, even in the absence of PP1, fully reactivated PP2A-B55 can dephosphorylate both Gwl-activating sites. On page 1340, Andrew Burgess and colleagues demonstrate that PP1 dephosphorylates and deactivates MASTL during mitotic exit in human cells. Furthermore, the deactivation occurs through the rapid dephosphorylation of Cdk1-directed Thr sites on MASTL. By using mathematical modelling, the authors confirm that deactivation of MASTL is essential for mitotic exit and show that PP1 is required to trigger the initial dephosphorylation of MASTL. PP2A inhibition is then released, completing MASTL and Cdk1 substrate dephosphorylation, and establishing a bistable switch that drives the exit from mitosis. Together, these two papers represent an important step forward in our understanding of how mitotic exit is regulated.