The development and subsequent approval of antibodies against the immune regulators programmed cell death receptor 1 (PD-1) and its ligand PD-L1 is changing treatment paradigms in a variety of cancers, with 5 of these antibodies approved to date.1 Whereas immune checkpoint inhibitor development is among the most successful and heavily investigated areas in oncology drug development, several uncertainties remain regarding how best to use these therapies in clinical practice.2 Important questions include how best to predict, identify, and optimally manage immune-related adverse events, the utility of complementary or companion diagnostic tests to predict which patients will and will not benefit, optimal dosing and scheduling, and the optimal duration of treatment.