Our immune system is capable of recognizing and eliminating cancer cells. This process is tightly regulated by immune checkpoints and their ligands. A major inhibitory immune checkpoint is programmed death-1 (PD-1), which is expressed by T-cells. Its ligand PD-L1 is expressed by tumor cells and antigen presenting cells (APCs). By upregulating PD-L1, tumor cells inactivate T-cells and escape immune recognition and attack. Clinical trials with anti-PD-1/PD-L1 immune checkpoint inhibitors (ICI) have shown durable (≥ 1 year) antitumor activities in 15-20% of cancer patients. This shows that ICI significantly improve the outcome for a subgroup of patients. However, a large number of non-responding patients is unnecessarily exposed to an expensive, ineffective treatment and its associated side effects. To use these expensive drugs more effectively, there is an urgent need for a biomarker to accurately predict treatment response. Tumor that express high levels of PD-L1 and PD-1 are most likely to respond to ICI. Therefore, we develop novel radiotracers to non-invasively image tumor PD-1 and PD-L1 expression by PET and SPECT. Recently, we have shown that it is feasible to distuingish high, moderate, and low PD-L1 expressing tumors by SPECT/CT imaging, using radiolabeled anti-PD-L1 antibodies.