Professor Sims-Williams recently completed a book on Irish influence on medieval Welsh literature which will be published by Oxford University Press in 2010. He also does research on the early Celtic languages, including Old Irish, Old and Middle Welsh, and the ancient Continental Celtic languages. He recently published a two-volume study of Roman inscriptions containing Celtic personal names, jointly with Marilynne E. Raybould (2007, 2009), and he is currently writing the linguistic commentary for the third and final volume of A Corpus of Early Medieval Inscribed Stones and Stone Sculpture in Wales (2007- ). In 2009 he delivered the John V. Kelleher Lecture in Harvard University on 'How our understanding of early Irish literature has progressed'. He has edited Cambrian Medieval Celtic Studies since 1981. He is a Fellow of the British Academy and directs the Academy's research project on the Development of the Welsh Language (Datblygiad yr Iaith Gymraeg).