Cancer cells and immune cells modulate their metabolism according to specific needs during cancer progression and immune responses. The ability to measure cellular metabolic function in vivo would enable the evaluation of tumors and their response to therapy and also the effectiveness of cellular immune responses to cancer. Positron emission tomography (PET) is a highly sensitive clinical imaging modality that enables whole-body, quantitative measurements of tissue biochemical function. Here, we review work using PET probes for specific metabolic pathways to measure cell function in cancer and immunity. We focus on the use of probes for glycolysis and nucleoside salvage and then discuss the development of new metabolic probes that visualize distinct parameters of cell function during disease.