This project examines a range of complex multilingual behaviours in sign language users and pursues three thematically related studies: a) Cross-signing : The development of improvised communication (ad hoc pidgins) between users of different sign languages in language contact situations; b) Sign-speaking : The simultaneous production of sign and speech, where the different structures of both languages are kept largely intact; and c) Sign-switching : Code-switching between sign languages in multilingual sign language users. None of these multilingual behaviours has ever been systematically investigated.The three studies use both lab-based experimental methodologies and discourse data from natural communicative situations. Subjects are drawn from a group of multilingual, mostly deaf, sign language users from various countries around the world. This project is situated at the crossroads between the domains of sociolinguistics, psycholinguistics, typological, and diachronic approaches to language. Together, the three focused studies break new ground and lay the foundation to a previously uncovered field of research that can be called sign multilingualism studies . This field arises when existing concepts of bi- and multilingualism are brought to bear on sign languages. Of particular interest are phenomena that are peculiar to situations involving sign languages, such as the rapid emergence of improvised inter-languages in cross-signing , or the simultaneous combination of conflicting syntactic structures in sign-speaking .In addition to the theme of sign multilingualism, the three sub-projects are also united by a particular interest in the meta-linguistic skills that the subjects use in both the experimental and the natural discourse settings. Some of these previously undocumented high-level skills take us right to the limits of linguistic abilities and have wider implications for our understanding of the human language faculty.