Abstract—This paper provides a comprehensive review of the cognitive optical networks as the complexity of optical networks has been increasing continuously in recent years. Based on our previous studies, we propose a novel framework of heterogeneous optical networks and introduce cognition to the optical networks in order to improve the overall network performance. Furthermore, this paper provides new insights on orthogonal frequency division multiplexing (OFDM) transmission technology, which is considered as a promising technology for future high speed optical transmission. Specifically, the solution to routing and spectrum allocation (RSA) problem in cognitive elastic OFDM-based optical networks is discussed in this paper. We also present possible challenges and propose new ideas to guide the future work of researchers in next generation optical networks in order to make future optical networks more cognitive and reconfigurable.