Cdk1 is an important cell cycle regulator that, in association with different cyclin regulatory subunits, is responsible for signaling important cell cycle events in all eukaryotic cells. In budding yeast, inhibition of Cdk1 by selective deletion of cyclin subunits has been shown to prevent anaphase onset, suggesting that Cdk1 activity is critically important for triggering anaphase onset. In many eukaryotes, Cdk1 has been shown to phosphorylate subunits of the anaphase promoting complex (APC), an E3 ubiquitin ligase which directly signals anaphase onset by triggering the degradation of the anaphase inhibitor securin. It is currently unclear, however, whether the APC is the sole essential substrate of Cdk1 in anaphase onset or if Cdk1 triggers anaphase onset by phosphorylating additional proteins. Eukaryotic Cdk1 is regulated by the Wee1 family of tyrosine kinases and the Cdc25 family of phosphatases which directly oppose Wee1 activity. Wee1 phosphorylation of Cdk1 on a single tyrosine residue inhibits Cdk1 and has been shown to prevent or delay mitotic entry. In this work we sought to further elucidate the mechanism through which Cdk1 regulates anaphase onset. We showed that, in addition to regulating mitotic entry, the budding yeast Wee1 kinase and Cdc25 phosphatase (Swe1 and Mih1 respectively in S. cerevisiae) regulate anaphase onset by modulating Cdk1 activity. Activation of Swe1 delays anaphase onset and cells lacking SWE1 enter anaphase prematurely, demonstrating that Swe1 regulates anaphase onset in unperturbed cell cycles. Deletion of the CDC55 regulatory subunit of PP2A has been shown to bypass cell cycle delays due to Swe1 activation. We showed that this is due, in part, to PP2ACdc55 dephosphorylation of Cdk1 sites on the APC. We have also shown that Cdk1 directly phosphorylates separase, the protease that dissolves sister chromatid linkages upon release from inhibitory securin/separase complexes upon APC-mediated securin degradation. Similar to phosphoregulation of the APC, we showed that Cdk1 phosphorylation of separase is opposed by PP2ACdc55. Phosphoregulation of separase appears to be important for regulation of the separase substrate Slk19 which cooperates with the conserved kinesin-5 Cin8 and microtubule bundling protein Ase1 to regulate spindle elongation at the spindle midzone.