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