— Current research networks allow end users to build their own application-specific connections (lightpaths) and Optical Private Networks (OPNs). This requires a clear communication between the requesting application and the network. The Network Description Language (NDL) is a vocabulary designed to describe optical networks based on the Resource Description Framework (RDF). These descriptions aid applications in querying the capabilities of the network and allow them to clearly express requests to the network. This article introduces NDL and shows its current applications in optical research networks.