The field of immuno-oncology has witnessed unprecedented success in recent 
years, with several PD=1 and PD-L1 inhibitors obtaining US FDA registration and 
breakthrough drug therapy designation in multiple tumor types. Despite its clear 
efficacy in certain cancers, treatment with these agents carries a risk of 
immune-related toxicities and substantial financial burden. It is, therefore, 
critical to identify patients likely to benefit from such immunotherapies and 
develop strategies to differentiate responders from nonresponders early during 
treatment. Here we discuss the development of predictive and treatment response 
biomarkers for immune checkpoint inhibitors. We first examine the role of PD-L1 
expression, the most extensively studied predictive biomarker of response, and 
further discuss emerging putative predictive biomarkers. We also detail 
challenges faced in the development of response assessments for 
immunotherapeutics and propose other biomarkers that may be useful as surrogate 
intermediate end points of response.