Molecular imaging allows the visual representation, characterization, and quantification of biological processes at the cellular and subcellular levels within intact living organisms. In oncology, it can be used to depict the abnormal molecules as well as the aberrant interactions of altered molecules on which cancers depend. Knowledge of the fundamental tissue, cellular, genomic, and molecular changes that form the hallmarks of cancer has led to the introduction of cancer therapies aimed at specific molecular targets. This chapter will illustrate why molecular imaging is an invaluable tool for developing and facilitating the appropriate use of such targeted treatments as well as conventional cancer treatments. Alone or combined with anatomic imaging, it is destined to play an increasingly important role in all stages of cancer care, from initial cancer detection through treatment and follow-up.