We investigate new routing and wavelength assignment algorithms considering as constraints physical impairments that arise in transparent networks. Accounting both linear and nonlinear impairments we propose a scheme that integrates the routing and wavelength assignment to achieve an optimal combination of physical and networking performance. Through simulations of a typical long haul network the improvement achieved using the proposed approach compared to conventional RWA algorithms (i.e shortest path and first fit) is demonstrated and the significance of impairment aware routing and wavelength assignment schemes is recognized.