“Spectrum defragmentation in elastic optical networks”.In elastic optical networks (EONs), dynamic setup and release (tear down) of lightpaths will lead to spreading the optical spectrum into nonaligned, isolated and small sized spectrum blocks. As introduced in previous chapters, this issue is called spectrum fragmentation. Defragmentation strategies are key counter measures to minimize the problem of spectrum fragmentation. Defragmentation, implemented by algorithms and usually running as defragmentation engine in the control plane, is a process that attempts to reduces the amount of fragmentation with the aim to reduce the blocking of new connection attempt and therefore allowing a higher utilization of network resource.Fragmentation management techniques deal with spectrum fragmentation and increase the amount of traffic that can be accepted and carried by a network.