Cancer immunotherapy treatments have shown promising results in several clinical trials and have demonstrated beneficial therapeutic outcomes even in very advanced stages of cancer. Recently, immune checkpoint inhibitor (ICIs)-based cancer immunotherapy treatments, such as: anti-CTLA-4, anti-PD-L1, and anti-PD-1 therapies, have been found to be highly effective as monotherapies and as a combination of cancer immunotherapies in many clinical trials. ICIs are antibodies or small molecules that specifically target and block the binding site of specific ligands on immune cells, thus strengthening the immune system’s response to cancer cells in the tumor microenvironment (TME). With the rapid rise of both number and popularity of the immune checkpoint blockade (ICB) cancer therapeutics, there has been an increasing need for reliable, robust, and noninvasive imaging approaches to measure in vivo immune checkpoint markers. Several immunoPET tracers that can be administered noninvasively to measure ICs in the TME have been developed to assist in the selection of appropriate patients for ICB treatments. In this chapter, we discuss the various immunotherapy strategies, including ICB therapy, and the corresponding immunoPET tracers used to evaluate and measure the IC markers to stratify the patients for suitable ICB therapy.