Molecular imaging is the visualization, characterization, and measurement of biologic processes at the molecular and cellular levels in humans and other living systems (1). It comprises an emerging set of technologies that builds on advances in imaging procedures (e.g., PET, SPECT, MRI, ultrasound, optical, and photoacoustic), improved understanding of biology, and the development of molecularly targeted agents. These continuously expanding sets of imaging methods are often used in combination, and advances in data acquisition and analyses facilitate a more complete understanding of biology. Molecular imaging aims to improve our understanding of mammalian biology and lead to advances in patient care by providing targeted therapies that will enable personalized medicine and the imaging tools to assess outcome. Implementation of these new technologies in clinical care has many educational, technical, and regulatory challenges that must be overcome before molecular imaging reaches its full potential. The impact of molecular imaging has been significant in several disciplines, because it represents a paradigm shift in how scientists and clinicians can observe biology in real time and in a relatively noninvasive manner to enable the power of repeated measures in living organisms. Copyright © 2011 the American Physiological Society.