The 2009 edition of the Amsterdam Colloquium was the 17th in a series which started in 1976. The Amsterdam Colloquia aim at bringing together linguists, philosophers, logicians and computer scientists who share an interest in the f- mal study of the semantics and pragmatics of natural and formal languages. Originally an initiative of the Department of Philosophy, the colloquium is now organized by the Institute for Logic, Language and Computation (illc)ofthe University of Amsterdam. These proceedings contain revised extended abstracts of most of the articles presented at the 17th Amsterdam Colloquium. The ?rst section containsextended abstractsof the talks givenby the invited speakers of the general program. The second, third and fourth sections contain invited and submitted contributions to the three thematic workshops that were hosted by the colloquium: the workshoponImplicature and Grammar organized by Maria Aloni and Katrin Schulz; the workshop on Natural Logic organized by Jan van Eijck; and the workshop on Vagueness, organized by Robert van Rooij and Frank Veltman. The ?nal section consists of the submitted contributions to the general program. For the organization of the 17th Amsterdam Colloquium ?nancial support was received from: the Royal Dutch Academy of Sciences (knaw); the Institute for Logic, Language and Computation (illc); thenwo-funded project 'Ind- inites and Beyond: Evolutionary pragmatics and typological semantics' (co- dinator: Maria Aloni); the nwo-funded project 'Vagueness - and how to be precise enough' (coordinators: Robert van Rooij and Frank Veltman); and the Municipality of Amsterdam. This support is gratefully acknowledged.