Biomedical optical imaging techniques have been instrumental in advancing biology and medicine as they offer powerful methods to image living specimen and dynamic biological processes. Furthermore, innovations in optical imaging techniques have continuously expanded their potential in terms of imaging resolution, depth, contrast, and biomedical applications. In this chapter, the history of microscopy development starting from the inception of the microscope to the expansion into nano-scale resolution imaging with super-resolution imaging techniques is briefly outlined. Various optical imaging techniques such as optical coherence tomography, Raman imaging, diffuse optical/correlation tomography, diffuse fluorescence tomography, and photoacoustic tomography are introduced. In addition, the trade-off between imaging resolution and depth among imaging techniques, the advantages of optical imaging over non-optical imaging, and the organization of the book are discussed.