With the tremendous growth of bandwidth-intensive networking applications, the demand for bandwidth over data networks is increasing rapidly. Wavelength division multiplexing (WDM) optical networks provide promising infrastructures to meet the information networking demands and have been widely used as the backbone networks in the Internet, metropolitan area networks, and high-capacity local area networks. Efficient routing on WDM networks is challenging and involves hard optimization problems. This chapter introduces efficient algorithms with guaranteed performance for fundamental routing problems on WDM networks.