This randomized, open-label study will evaluate the efficacy and safety of atezolizumab compared with chemotherapy consisting of a platinum agent (cisplatin or carboplatin per investigator discretion) combined with either pemetrexed (non-squamous disease) or gemcitabine (squamous disease) in programmed death-ligand 1 (PD-L1)-selected, chemotherapy-naive participants with Stage IV Non-Squamous or Squamous NSCLC.