Biomarkers are indicators of specific biological processes occurring in vivo. Basically, they are things we can measure that let us keep track of something physiologically important. For example, the size of a tumor is a simple biomarker for cancer, and capillary density of plaques in blood vessel walls are biomarkers for atherosclerosis. The advent of molecular imaging lets us study biomarkers in a mostly noninvasive fashion in living animals, and for a small number of biomarkers, even in people. Imaging comes in many modalities, depending on the application; these include magnetic resonance imaging (MRI), positron emission tomography (PET), single photon emission computed tomography (SPECT), autoradiography and optical and ultrasound imaging. Here is a look at some important uses of molecular imaging and biomarkers todayâ€”as bioresearch tools with potential clinical value.