Optical networks employing Wavelength Division Multiplexing (WDM) technology allow the multiplexing of several independent wavelength channels into a fiber. Since each wavelength channel operates independently at several Gb/s, WDM optical networks offer a tremendous transport capacity (which is in the order of several Tb/s), which makes them suitable candidates for future networks. A lightpath is made up of wavelength channels between the source and destination nodes to transfer a large amount of data. Routing in WDM networks involves assigning both paths and wavelengths, and is called routing and wavelength assignment (RWA). In WDM optical networks, there are two vital RWA issues that have garnered a lot of interest from researchers as well as network operators. 1) Survivability: Lightpaths in WDM networks usually transport a tremendous amount of data. If a lightpath fails due to various natural or man-made disasters, the data loss can be costly. Hence, survivability, which is the ability to reconfigure and resume communication is indispensable. 2) Impairment-aware routing: As an optical signal traverses its path, it encounters noise and signal distortions along its way. These physical impairments cause bit errors, which may make the signal unrecognizable at the receiving end. In order to reverse the effect of physical impairments, the signal needs to be regenerated at intermediate nodes. Unlike traditional RWA, impairment-aware RWA, takes into account the effect of physical impairments. Impairment-aware RWA entails two important issues: a) Impairment-aware path selection: how to find a feasible path from the source to the destination node? b) Regenerators placement: how many regenerators are required and where to place them in the network? The main focus of this thesis is to study various problems associated with survivability and impairment-aware RWA both in intra-domain (i.e., within in a single domain) and inter-domain (i.e., across domains) WDM networks. Especially, for intra-domain networks, it makes a detailed study of survivable and impairment-aware RWA issues, separately or combined. For the various problems studied in this thesis, the complexity of the problems is analyzed in detail, and accordingly exact, approximation or heuristic algorithms are proposed for solving them. In addition, a case study of survivable and impairment-aware routing is made on a realistic network that connects research and educational institutes in the Netherlands using data obtained from the network. The work done in this thesis will not only help us gain insight into the various problems in WDM networks, but it may also be applicable to corresponding problems in other types of networks, or even to problems in other areas.