This title intends to describe principles of optical imaging including chemistry and physics of fluorescence, limitations/advantages of optical imaging compared to metabolic and anatomic imaging. It describes hardware adapted for small animal imaging and for clinical applications: endoscopes and operative microscopes. It outlines FDA approved and newer optical imaging probes. It includes discussion of chemistry and linkage to other proteins. It reviews current techniques to image cancer and the development of techniques to specifically image cancer cells. It reviews use of exploiting differences in tissue autofluorescence to diagnose and treat cancer. Include agents such as 5-aminoleculinic acid. It reviews mechanisms that require proteolytic processing within the tumor to become active fluorophores. It reviews use of cancer selective proteins to localize probes to cancer cells: include toxins, antibodies, and minibodies. Introduction of plasmids, viruses or other genetic material may be used to express fluorescent agents in vivo. This chapter will review multiple vectors and delivery mechanisms of optical imaging cassettes. Preclinical investigations into the use of optical contrast agents for the detection of primary tumors in conventional and orthotopic models will be discussed. Preclinical investigations into the use of optical contrast agents for the detection of metastatic tumors in mouse models will be discussed. Use of targeted and non-specific optical contrast agents have been used for the detection of sentinel lymph node detection. These applications and how they differ from other applications will be discussed. Because of the unique difficulty of identifying tumor from normal tissue in brain tissue, a separate chapter would be needed. More clinical data is available for this cancer type than any other. There is a discussion of potential clinical applications for optical imaging and an assessment of the potential market.