The simple idea of having IP traffic directly transported over optical Wavelength Division Multiplexing (WDM) technology (“Internet Protocol (IP) over WDM”) has been envisioned as the future for the telecommunication infrastructure. The key point in this infrastructure becomes the switching layer between ubiquitous IP-centric networks and WDM physical layer. An intense debate has been ongoing about which model to adopt, aiming at identifying the degree of optical transparency and the proper flexibility of optical interconnection. Expected migration of switching functions from electronic to optics will be gradual, and will take place in several phases. The first phase is presented in this chapter. It is characterised by the use of the wavelength as a switching granularity; the terms optical circuit switching (Optical Circuit Switching (OCS)), wavelength switching and wavelength routed network are commonly and indifferently used.