The 27th International Conference on Optical Network Design and Modelling (ONDM 2023) will take place at the Department of Mathematics, of the University of Coimbra, Coimbra, Portugal from 8th May to 11th May 2023. The conference will be organized by INESC Coimbra, Coimbra, Portugal, and is planned to include workshops, keynotes, tutorials, and invited talks, as well as high-quality contributed talks. The conference is expected to be highly interactive and to be a means to disseminate active and emerging areas of optical networking. Following the tradition of past ONDM conferences, ONDM 2023 will address cutting-edge research in established areas of optical networking and their adoption in support of a wide variety of new services and applications. This includes the most recent trends such as 5G and beyond; data-center networking; Internet of things; cloud/edge computing; content delivery; big data, data analytics, network telemetry, and real-time monitoring; autonomic networking; artificial intelligence/machine learning assisted networks; visible light networks; and quantum secured networks.