In this chapter, we gave an overview of the key elements of modern optical networking. Although the theoretical underpinnings are rather specialized [Ramaswamy+02], it is useful to have a high-level understanding of the technology to appreciate the networking and control plane concepts that follow in later chapters. Our review in this chapter is by no means exhaustive. Readers may refer to [Ramaswamy+02, Stern+99, Mukherjee97] for in-depth treatment of optical network architecture. For a thorough treatment of optical transmission systems including DWDM, please refer to [Agrawal97, Green92]. For a detailed discussion on optical switching, please refer to [Mouftah+98, Hinton93].