In this paper we overview our research activities on the Routing and Spectrum Assignment (RSA) in optical metro ring networks with flexible grid and distance-adaptive transceivers. We consolidate our existing mathematical models of the RSA problem in a single comprehensive formulation that can work under very diverse assumptions (variable-vs. fixed-spectrum transceivers, regeneration vs. no regeneration, grooming vs. no grooming, etc.). We discuss the complexity of the various versions of the RSA problem and summarize the main numerical results obtained in our previous works. A final section is dedicated to an overview of possible extensions of this work in the context of Space Division Multiplexing.