Modern optical transport networks consist primarily of optical switches interconnected using Dense Wavelength Division Multiplexing (DWDM) transmission systems. In this chapter, we review the basic characteristics of optical transmission and switching systems. Specifically, we describe the working principles that guide the generation and reception of optical signals and its transmission through the optical fiber. We also review the technological foundation behind optical switching, an important element of modern optical transport networks. The discussion in this chapter highlights the physical properties of optical networks, many of which may appear as constraints in the control plane.