Structural imaging is currently used in standard clinical practice on a daily basis to qualitatively or semiquantitatively detect, characterize stage, assess posttherapeutic change in, and determine recurrence of malignant tumors based on structural features or gross degree of contrast enhancement. Unfortunately, structural imaging does not provide information about tumor physiology, biological processes, and molecular features, and as such tumors cannot be fully characterized and monitored. In order to improve the evaluation of tumors and to reduce cancer-related morbidity and mortality, there is a need for functional imaging modalities which allow visualization and quantification of physiological and biochemical processes in vivo. This article will review a selection of the wide variety of functional imaging methods available for non-invasive evaluation of tumor physiology and molecular processes.