This document discusses license interoperability and the lack thereof between different license models. It notes that public domain and Creative Commons Attribution licenses allow for the highest degree of interoperability, while share-alike licenses and those with non-commercial clauses present problems and legal uncertainty. National open data licenses also attempt to address interoperability but still face issues around privacy, trademarks and other concerns beyond just copyright. Overall license complexity and the interaction of copyright with other laws pose challenges to achieving full legal interoperability between works.