This volume of the Proceedings of the Harvard Celtic Colloquium offers a wide range of articles on topics across the field of Celtic Studies. It includes the 2017 J. V. Kelleher lecture delivered by Paul Russell, Professor of Celtic, University of Cambridge, entitled "'Mistakes of All Kinds': The Glossography of Medieval Irish Literary Texts." In this address Russell offers cogent analysis of this rarely addressed facet of medieval Irish codicology. The articles from other presentations at the Colloquium extend the focus on Celtic glossing into other areas of Celtic linguistics and literary studies. In addition, the volume includes articles on the medieval folkloric, religious, legal, and material culture of Celtic communities, some aspects of which persist into modernity. This volume exemplifies the broad range of topics and time periods characteristic of the Harvard Celtic Colloquium.