This volume constitutes the proceedings of the 21st International Conference on Theorem Proving in Higher Order Logics (TPHOLs 2008), which was held during August 18-21, 2008 in Montreal, Canada. TPHOLs covers all aspects of theorem proving in higher order logics as well as related topics in theorem proving and veri?cation. There were 40 papers submitted to TPHOLs 2008 in the full research c- egory, each of which was refereed by at least four reviewers selected by the ProgramCommittee. Of these submissions, 17 researchpapers and 1 proofpearl were accepted for presentation at the conference...