  This volume contains the proceedings of EXPRESS/SOS 2021: the Combined 28th
International Workshop on Expressiveness in Concurrency and the 18th Workshop
on Structural Operational Semantics, which was held online, as an affiliated
workshop of CONCUR 2021, the 32nd International Conference on Concurrency
Theory. The EXPRESS/SOS workshop series aims at bringing together researchers
interested in the formal semantics of systems and programming concepts, and in
the expressiveness of computational models.
