Modern pharmaceuticals and medical devices have provided substantial benefits to patients throughout the world. These benefits come at a high and increasing cost, with development of the typical pharmaceutical requiring 12 years and hundreds of millions of dollars before gaining U.S. Food and Drug Administration marketing approval. Appropriate use of imaging biomarkers--defined as anatomic, physiologic, biochemical, or molecular parameters detectable with imaging methods used to establish the presence or severity of disease--offer the prospect of smaller, less expensive, and more efficient preclinical studies and clinical trials. Scientists, government regulators, and industry have all recognized the potential of biomarkers in imaging. Although real, this promise can only be realized with the rigorous application of science to their use. Success is most likely when (a) the presence of an imaging marker is closely linked with the presence of a target disease; (b) detection and/or measurement of the biomarker is accurate, reproducible, and feasible over time; and (c) measured changes are closely linked to success or failure of the therapeutic effect of the product being evaluated. By applying this paradigm to the array of imaging modalities, the radiology community is poised to become a major force in preclinical and clinical evaluations of new medical treatments.