 This multicenter, randomized, double-blind study will evaluate the efficacy, safety, and pharmacokinetics of atezolizumab MPDL3280A administered with nab-paclitaxel compared with placebo in combination with nab-paclitaxel in participants with locally advanced or metastatic triple-negative breast cancer TNBC who have not received prior systemic therapy for metastatic breast cancer mBC. The safety of single-agent nab-paclitaxel has been determined in previous studies of participants with mBC and the safety data to date suggest that atezolizumab can be safely combined with standard chemotherapy agents. 