Molecular imaging provides a visualization of normal as well as abnormal cellular processes at a molecular or genetic level rather than at the anatomical level. Molecular imaging is rapidly emerging and a multidisciplinary field coordinating medicine, molecular cell biology, chemistry, pharmacology, genetics, biomedical engineering, and physics. Conventional medical imaging methods utilize the imaging signals produced by nonspecific physico-chemical interaction. However, molecular imaging methods utilize the imaging signals derived from specific cellular or molecular events. Because molecular and genetic changes precede anatomical change in the course of disease development, molecular imaging can detect early events in disease progression. Molecular imaging includes images of proteomics, metabolism, cellular biologic processes as well as genetics. In a narrow sense, molecular imaging means genetic imaging using imaging reporter genes. We can image diverse cellular processes including gene expression, protein-protein interaction, signal transduction pathway, and monitoring of target cell distribution (cancer cells, immune cells, and stem cells) by imaging reporter gene. Molecular imaging methods are classified as optical imaging, nuclear imaging and magnetic resonance imaging. Each imaging modalities have their advantages and weaknesses. In the near future, through molecular imaging we can understand basic mechanisms of disease, and diagnose earlier and, subsequently, treat earlier intractable diseases such as cancer, neuro-degenerative diseases, and immunologic disorders.