Given the increasing and accelerating demand for higher bandwidth per user, an evolution from the currently deployed passive optical networks (PONs) to next-generation optical access networks (NG-OANs) is becoming inevitable. In addition to fulfilling the bandwidth requirement, the NG-OANs should overcome the limitations of existing gigabit-class PONs, such as low capacity, limited reach, and restricted mobility. Different architectures for creating a NG-OAN have been proposed in the literature; however, each architecture sought to overcome only one of the aforementioned limitations. In this paper, the authors provide a review of these state-of-the-art architectures, including the contributions and challenges of each. A comparison evaluating key network specifications is also provided.