This Phase Ib-IIa, multi-institutional, open-label, dose-escalation study is designed to evaluate the safety, tolerability, pharmacokinetics and feasibility of trastuzumab emtansine (T-DM1) administered by intravenous (IV) infusion in combination with paclitaxel (and pertuzumab, if applicable) in patients with human epidermal growth factor receptor 2-positive (HER2-positive), locally advanced or metastatic breast cancer.