Blocking the programmed cell death 1 (PD-1) pathway with monoclonal antibodies has shown promising antitumor responses in clinical trials, with less toxicity than has been seen with prior immune therapies such as interleukin 2 and ipilimumab. Pembrolizumab, an anti-PD-1 antibody, recently gained US Food and Drug Administration (FDA) accelerated approval for the treatment of patients with ipilimumab-refractory melanoma, while nivolumab, another anti-PD-1 antibody, and MPDL3280A, an anti-programmed cell death 1 ligand (PD-L1) antibody, have been granted FDA "breakthrough designation" for treatment of subsets of patients with refractory Hodgkin lymphoma and metastatic bladder cancer, respectively. Encouraging antitumor activity has also been seen with these agents in patients with other malignancies, including non-small-cell lung cancer and head and neck cancer, tumors not previously thought to be immune-responsive. PD-L1 expression has emerged as a potential predictive biomarker for PD-1-directed therapy. Multiple, distinct, companion assays for PD-L1 positivity have been developed, but there is as yet no comparison, standardization, or prospective validation of these assays. PD-L1 expression on tumor cells and/or the tumor-immune infiltrate is likely only part of the predictive model necessary for selecting patients predisposed to respond to monotherapy. Additional predictive biomarkers are necessary to identify patients most likely to benefit from PD-1-based combination therapy, since tumor cell PD-L1 expression appears to have limited predictive value in this setting.