This study is mainly concentrate on the routing problem in optical WDM network. In WDM network, the wavelength continuity constrain must be taken care in data communication. Lightpath is the communication channel between each pair of nodes. The lightpath is set up on basis of connection request, in such a way that no two ligthpaths shares the same link. The routing is based on the available wavelength and it assignment. This study focuses on the routing problem in wavelength routed WDM networks which works under wavelength continuity constrain. Various routing methods discussed are fixed routing, fixed alternate routing, adaptive routing, adaptive alternate routing and fault tolerance along with the blocking probability. The connection requests have been rejected, if there is lack of availability of resources for the requested link is known as blocking probability.