A multitude of noninvasive, quantitative, functional imaging techniques are currently in use to study tumor physiology, to probe tumor molecular processes, and to study tumor molecules and metabolites in vitro and in vivo using computed tomography (CT), magnetic resonance imaging (MRI), ultrasonography (US), positron emission tomography (PET), single-photon emission computed tomography (SPECT), and optical imaging (OI). Such techniques can be used in conjunction with structural imaging techniques to detect, diagnose, characterize, or monitor tumors before and after therapeutic intervention. These can also be used to study tumor gene expression, to track cells and therapeutic drugs, to optimize individualized treatment planning for patients with tumors, and to foster new oncologic drug development. In this article, we review the rich variety of functional imaging techniques that are available for these purposes, which are becoming increasingly important for optimal individualized patient treatment in this day and age of "personalized medicine."