In an optical wavelength division multiplexed (WDM) network, link or node failures may result in huge amounts of lost data, due to the enormous fiber throughput. This requires that optical WDM network be designed to be resilient to failures. Thus, survivability can be defined as the ability to respond gracefully to such failures. This chapter presents a comprehensive survey of various mechanisms proposed to achieve survivability. The survey considers different topologies, different failure models, implementation issues, signaling issues and quality-of-protection issues.