The present invention provides therapeutic and diagnostic methods and compositions for cancer, for example, non-small cell lung cancer (NSCLC). The invention provides methods of treating NSCLC, methods of determining whether a patient suffering from NSCLC is likely to respond to treatment comprising a PD-L1 axis binding antagonist, methods of predicting responsiveness of a patient suffering from NSCLC to treatment comprising a PD-L1 axis binding antagonist, and methods of selecting a therapy for a patient suffering from NSCLC, based on expression levels of a biomarker of the invention (e.g., PD-L1 expression levels in tumor cells and/or tumor-infiltrating immune cells).
