Positron emission tomography (PET) is one of the most rapidly growing areas of medical imaging for cancer research. The principal goal of PET imaging is to visualize, characterize, and measure biological processes at the molecular and cellular levels in living subjects using non-invasive procedures. Taking advantage of the traditional diagnostic imaging techniques, PET imaging introduces positron-emitting probes to determine the expression of indicative molecular targets at different stages of cancer progression. As a key component of PET technique, an appropriate imaging probe must be able to specifically reach the target of interest in vivo while retaining in the target within reasonable time to be detected. Over the last decade, numerous target-specific PET probes have been developed and evaluated in preclinical and clinical studies. This review provides an overview of recent advances made in PET imaging of cancer biology with a focus on the best-studied biological targets. The trends in developing future PET imaging probes are also discussed.