There has been a growing awareness in the NLP community of the problems that multiword expressions (MWEs) pose. Developments in areas such as machine translation, text summarization, paraphrasing, grammar development and parsing, information retrieval, and question answering (to mention a few) have acknowledged difficulties due to the idiosyncratic nature of multiword expressions. This workshop continues a tradition of ACL workshops on Collocations (2001) and Multiword Expressions (2003 and 2004). Its specific objective is to focus on the underlying properties of MWEs. The call for papers expressed our interest in several topics such as the definition of MWEs, properties of MWEs and their impact on NLP applications, representation and treatment of the different classes of MWEs, linguistic and psycholinguistic analyses of MWEs, evaluation of extraction techniques and the importance of (non-)compositionality.