In vivo imaging in small animals is playing an increasingly important role in the understanding of basic physiological processes, in the study of disease and evaluation of therapies; in addition to providing unique information in its own right, it also provides an essential bridge between invasive techniques, which often require postmortem analysis, and noninvasive approaches that are applicable to studies in human subjects. A large number of in vivo imaging techniques have become available in the last decades including optical bioluminescence/fluorescence, single photon emission computed tomography (SPECT), positron emission tomography (PET), computed tomography (CT) and ultrasound (US), magnetic resonance imaging (MRI) in its various flavors, such as magnetic resonance spectroscopy (MRS), diffusion tensor imaging (DTI), functional MRI (fMRI), and others.