Abstract: In this paper, an overview of research on the IP over wavelength division multiplexing networks using optical path switching is presented. Three aspects of the design are emphasized. The first is the design of virtual topology, including protection design and reconfiguration algorithms. The second is on the algorithms for traffic grooming. An optical wavelength can provide huge bandwith, but the transmission rates of the services are usually much lower. Low rate services should be appropriately merged into the wavelengths in the network, which is called traffic grooming, to obtain good network performance and lower cost. The third is on the network survivability. Finally, problems for further research are discussed.