The Automatically Switched Optical Networks (ASONs) and the Generalized Multi Protocol Label Switching (GMPLS) control plane are envisaged to play an important role in the next generation Internet. They provide optical networks with intelligence and automation and they enable the Multi-layer Traffic Engineering (MTE) paradigm. In this chapter we review different aspects of Multi-layer Traffic Engineering that have been investigated in COST action 291. We introduce integrated routing schemes and show fundamental performance metrics. Furthermore, we discuss mechanisms for multi-layer traffic engineering beyond routing that either improve the overall performance of the network or increase the fairness among different users. Finally, we summarize results of an performance evaluation that shows the impact of fundamental traffic and network characteristics on the performance.