The need to establish wavelength-routed connections in a service-differentiated fash­ ion is becoming increasingly important due to a variety of candidate client networks (e. g. IP, SDH/SONET, ATM) and the requirements for Quality-of-Service (QoS) de­ livery within transport layers. Up until now, the criteria for optical network design and operation have usually been considered independently of the higher-layer client signals (users), i. e. without taking into account particular requirements or constraints originating from the users' differentiation. Wavelength routing for multi-service net­ works with performance guarantees, however, will have to do with much more than finding a path and allocating wavelengths. The optimisation of wavelength-routed paths will have to take into account a number of user requirements and network con­ straints, while keeping the resource utilisation and blocking probability as low as pos­ sible. In a networking scenario where a multi-service operation in WDM networks is assumed, while dealing with heterogeneous architectures (e. g. technology-driven, as transparent, or regenerative), efficient algorithms and protocols for QoS-differentiated and dynamic allocation of physical resources will playa key role. This work examines the development of multi-criteria wavelength routing for WDM networks where a set of performances is guaranteed to each client network, taking into account network properties and physical constraints.