His expertise is in the fields of the history and language of Wales and Ireland, during the so-called Irish Dark Age (during the Roman Empire) and the general "Dark Ages", which followed the collapse of the Roman Empire in the west.
Charles-Edwards, Thomas (1971). "The Date of the Four Branches of the Mabinogi". Transactions of the Honourable Society of Cymmrodorion (1971, Part 2): 263–298. hdl:10107/1418639.
— (1978). "Honour and Status in some Irish and Welsh Prose Tales". Ériu. 29. Royal Irish Academy: 123–141. JSTOR30007769.
— (1978). "The Authenticity of the Gododdin: An Historian's View". In Bromwich, R.; Brinley Jones, R. (eds.). Astudiaethau ar yr Hengerdd: Studies in Old Welsh Poetry, cyflwynedig i Syr Idris Foster. Cardiff: University of Wales Press. pp. 44–71. ISBN0708306969.
—; Kelly, Fergus, eds. (1983). Bechbretha: an Old Irish law-tract on bee-keeping. Early Irish Law Series. Vol. 1. Dublin Institute for Advanced Studies. ISBN9781855002098.
—; Owen, Morfydd; Walters, D. B., eds. (1986). Lawyers and Laymen: Studies in the History of Law presented to Professor Dafydd Jenkins on his seventy-fifth birthday. Cardiff: University of Wales Press. ISBN0708309259.
— (1989). The Welsh Laws. Writers of Wales. Cardiff: University of Wales Press. ISBN070831032X.
— (1991). "The Arthur of History". In Bromwich, R.; Jarman, A. O. H.; Roberts, B. F. (eds.). The Arthur of the Welsh: The Arthurian legend in Medieval Welsh Literature. Arthurian Literature in the Middle Ages. Vol. 1. Cardiff: University of Wales Press. pp. 15–32. ISBN0708313078.
—, ed. (25 March 1993). Early Irish and Welsh Kinship. Oxford: Clarendon Press. ISBN9780198201038.
— (30 November 2000). Early Christian Ireland. The Cambridge History of Ireland. Cambridge University Press. ISBN0521363950.
—; Owen, Morfydd; Russell, Paul, eds. (April 2002). The Welsh King and His Court. Cardiff: University of Wales Press. ISBN9780708316276.
—, ed. (13 November 2003). After Rome. Short Oxford History of the British Isles. Oxford University Press. ISBN9780199249824.
—; Russell, Paul, eds. (2005). Tair Colofn Cyfraith. The Three Columns of Law in Medieval Wales: Homicide, Theft and Fire. Vol. 5. Bangor: Welsh Legal History Society. ISBN0954163745.