This is a phase 1B inter-patient dose escalation study of PD-0332991 in combination with T--DM1 in patients with recurrent or metastatic HER2-positive breast cancer after prior trastuzumab or other HER2-directed therapies. A standard 3+3 trial design will be used for PD-0332991 dose escalation cohorts. The 3+3 design entails that if one patient out of the first three patients has a dose-limiting toxicity (DLT), three additional patients will be entered at that dose level. The PD-0332991 dose levels will start at 100 mg po daily; the second cohort will receive 150mg po daily; the third cohort 200mg po daily. Patients receive PD-0332991 on days 5-18 of each 21 day cycle. T-DM1 will be given intravenously at 3.6 mg/kg on day 1 of each 21 day cycle.