The recent emergence of “molecular imaging” as an integrated discipline in academic medical centers has set the stage for an evolutionary leap in diagnostic imaging and therapy. 1 Molecular imaging is not a substitute for the traditional process of image formation and interpretation, but is intended to improve diagnostic accuracy and sensitivity by providing an in vivo analog of immunocytochemistry or in situ hybridization. It is less concerned with native image contrast or resolution, which are keys for depicting the effects of the disease on surrounding normal tissues, but rather, it seeks to enhance the conspicuity of microscopic pathologies by targeting the molecular components or processes that represent actual mechanisms of disease. Moreover, imaging will become crucial for in vivo characterization of the complex behaviors of disease in time and space that will tell us: where it is, how big it is, how fast it is developing, how many molecular processes are contributing simultaneously, what to treat it with, how it is responding to therapy, and how it is changing. Elements fundamental to the growth of molecular imaging are exemplified in summary statements from the second Biomedical Imaging Symposium: Visualizing the Future of Biology and Medicine (Available at: http://www. becon1. nih. gov/symposium1999. htm), which convened at the National Institutes for Health (NIH) on June 25 and 26, 1999, and was cosponsored by the Biomedical Engineering Consortium (BECON), the Radiological Society of North America, and the American Institute of Medical and Biological Engineering. The goals of the Symposium were to:(1)“identify the most important challenges and opportunities in biomedical imaging science,” and (2)“develop strategies for integrating imaging science with biological and medical research.” The formation of the BECON was followed by a Congressional mandate to establish a new National Institute for Biomedical Imaging and Bioengineering devoted to the development of novel technologies, including molecular imaging and therapeutics, without the traditional restrictions of hypothesis-driven research. The BECON symposium identified 5 action items critical to achieving these goals: