  This volume contains the proceedings of the 18th International Conference on
Quantum Physics and Logic (QPL 2021), which was held June 7-11, 2021 at the
University of Gdansk and online. QPL is an annual conference that brings
together researchers working on mathematical foundations of quantum physics,
quantum computing, and related areas, with a focus on structural perspectives
and the use of logical tools, ordered algebraic and category-theoretic
structures, formal languages, semantical methods, and other computer science
techniques applied to the study of physical behaviour in general.
