Molecular imaging is a rapidly emerging field that translates many concepts developed for molecular biology and cellular imaging to the in vivo imaging of intact organisms. The technique allows the study of molecular biological events in their full context and will therefore become an indispensable tool for biomedical research and drug discovery and development. This volume familiarizes the reader with the concepts of imaging and molecular imaging in particular. Basic principles of imaging technologies, reporter moieties for the various imaging modalities and the design of target reporter constructs are described in the first part. The second part illustrates how these tools can be used to visualize relevant molecular events: the biodistribution of drugs/ligands, the expression of drug targets (receptors, enzymes), and the consequences of the molecular drug-target interactions (pathway activations, system responses). A final chapter deals with visualization of cell migration (cell therapies).