Scholars have long believed that William Langland had a technical knowledge of the law. Piers Plowman: A Glossary of Legal Diction is the first attempt to confirm that belief through a comprehensive study of the poet's language. The Glossary examines more than 600 words and phrases (from all three versions of the poem) that had legal significance in 14th-century England. Entries include not only such specialized terms as escheat, mainprise, and supersedeas, but also words like challenge, determine, farm, occupy, and reason, whose legal significance has been obscured by centuries of ordinary usage. Each entry consists of a definition, one or more examples in Piers Plowman, and supporting citations from legal sources. An introductory essay places the poet's use of legal terminology within the larger framework of his moral purpose.