A Study Evaluating the Efficacy and Safety of Adjuvant Atezolizumab or Placebo and Trastuzumab Emtansine for Participants With HER2-Positive Breast Cancer at High Risk of Recurrence Following Preoperative Therapy

Inclusion Criteria:

* Histologically confirmed invasive breast carcinoma
* Centrally-confirmed human epidermal growth factor receptor 2 (HER2)-positive invasive breast cancer
* Centrally confirmed PD-L1 and hormone receptor status
* Clinical stage at disease presentation (prior to neoadjuvant therapy): cT4/anyN/M0, any cT/N2-3/M0, or cT1-3/N0-1/M0 (participants with cT1mi/T1a/T1b/N0 are not eligible)
* Completion of pre-operative systemic chemotherapy including at least 9 weeks of taxane and 9 weeks of trastuzumab (anthracycline and/or additional HER2-targeted agents are permitted)
* \<=12 weeks between primary surgery and randomization
* Eastern Cooperative Oncology Group (ECOG) Performance Status 0 or 1
* Screening left ventricular ejection fraction (LVEF) \>= 50% and no decrease in LVEF by \>15% from the pre-chemotherapy LVEF. If no pre-chemotherapy LVEF, screening LVEF \>= 55%
* Life expectancy \>= 6 months
* Adequate hematologic and end organ function

Exclusion Criteria:

* Stage IV breast cancer
* An overall response of disease progression according to the investigator at the conclusion of preoperative systemic therapy
* Prior treatment with T-DM1, or atezolizumab, or other immune checkpoint inhibitors
* History of exposure to various cumulative doses of anthracyclines
* History of other malignancy within 5 years prior to screening, except for appropriately treated carcinoma in situ of the cervix, non-melanoma skin carcinoma, Stage I uterine cancer, or ductal carcinoma in situ (DCIS)
* Current grade \>=2 peripheral neuropathy
* History of idiopathic pulmonary fibrosis, organizing pneumonia, or pneumonitis
* History of or active autoimmune disease or immune deficiency
* Treatment with immunostimulatory or immunosuppressive agents
* Cardiopulmonary dysfunction
* Any known active liver disease