PET/CT imaging has been applied to a variety of physiologic processes and disease states. However, this technology has arguably made its greatest impact on the field of oncology, where biochemical changes occur before changes in anatomy, promising early localization of disease for optimal management. PET/CT technology entails the visualization, characterization, and quantification of various biological processes occurring during tumorigenesis with different levels of analysis. Thus, it has been imposed in clinical oncological practice over the last two decades and has the potential to play a key role in the transition to precision medicine. In this chapter, we review PET imaging of selected aspects of tumor biology along with the emerging role of artificial intelligence (AI) in the field of molecular imaging.