  This volume contains the proceedings of the ninth workshop on Quantum Physics
and Logic (QPL2012) which took place in Brussels from the 10th to the 12th of
October 2012.
  QPL2012 brought together researchers working on mathematical foundations of
quantum physics, quantum computing, and spatio-temporal causal structures. The
particular focus was on the use of logical tools, ordered algebraic and
category-theoretic structures, formal languages, semantical techniques, and
other computer science methods for the study of physical behaviour in general.
