In this part, we discuss and describe in great detail various switching techniques for optical wide area networks (WANs). A number of different optical switching techniques have been proposed for backbone wavelength division multiplexing (WDM) networks over the last few years. Our overview will focus on the major optical switching techniques that can be found in today's operational long-haul WDM networks or are expected to be likely deployed in future optical WANs. In our overview we do not claim to provide a comprehensive description of all proposed switching techniques. Instead, we try to focus on the major optical switching techniques and describe their underlying principles and operation at length. We believe that our overview of carefully selected optical switching techniques fully covers the different types of switching techniques available for optical WANs and helps the reader gain sufficient knowledge to anticipate and understand any of the unmentioned optical switching techniques that in most cases might be viewed as extensions or hybrids of the optical switching techniques discussed. For instance, a so-called light-trail is a generalization of a conventional point-to-point lightpath in which data can be dropped and added at any node along the path, as opposed to a lightpath where data can be added only by the source and dropped only by the destination node, respectively (Gumaste and Zheng, 2005). Another good example is fractional lambda switching (FλS) (Baldi and Ofek, 2002). FλS uses the globally available coordinated universal time (UTC) as a common time reference to synchronize all optical switches throughout the FλS network.