We review the most important WDM DCI system innovations. Stateof-the-art coherent WDM transmission has already exceeded 6 b/s/Hz, using subcarrier modulation. Moreover, the adoption of software innovations in automation and programmability, that DCI pioneered in transport networks, has also simplified operations and enables the emergence of "open" transport architectures. Combining these advancements with emerging network analytics frameworks allows exciting innovations in network design and management optimization.