The Opus arduum valde is a Latin commentary on the Book of Revelation, written in England by an unknown scholarly author in the years 1389-1390. The book originated from the early Wycliffite movement and reflects its experience of persecution in apocalyptic terms. In England it soon fell into oblivion, but was adopted by radical exponents of the fifteenth-century Bohemian Hussites. In the sixteenth century Luther obtained a copy of the Opus arduum valde which he had printed in Wittenberg with his own preface in 1528. This remarkable document of religious dissent in late medieval Europe, highly regarded in Lollard and Hussite studies, is now for the first time made available in a critical edition.