Flexible grid optical networks (FGONs), a.k.a. elastic optical networks, have attracted intensive research interest due to its advantage over the traditional wavelength division multiplexing optical networks in accommodating diverse services with high spectral flexibility and utilization efficiency. Sophisticated routing and spectrum assignment (RSA) algorithms are the key enabling technologies to realize the flexibility and the efficiency in FGONs, but most of these RSA algorithms neglected some small-sized, isolated spectral bands generated in the spectrum allocation procedures. These spectral bands, known as spectrum fragments, are caused by the continuity and the contiguity constraints in spectrum allocation and can hardly be utilized by the successive service requests. With the accumulation of spectrum fragments, the available resources of FGONs can be exhausted and their networking performance will be greatly affected. Therefore, spectrum fragmentation has become an important issue in FGONs, and many defragmentation algorithms, including both the preventive and the reactive spectrum defragmentation algorithms, have been proposed to reduce spectrum fragments. In this paper, we will review the current defragmentation algorithms for FGONs and analyze the characteristics of these algorithms first. Given the fact that most of the current defragmentation algorithms were designed for the point-to-point services in FGONs, we will then address the issue of designing the defragmentation algorithms for other kinds of services (e.g. multicast services) in FGONs. After that, we will demonstrate a spectrum defragmentation algorithm for multicast services in FGONs together with its simulation results.