Dissatisfaction with the state of the Church for one reason or another is as old as Christianity itself, but the charge that it was riches which had deflected it from the course laid down in the apostolic age only surfaced in the eleventh century. The impact of this idea from its beginnings in the eleventh century up to the Reformation is the theme of this book. Here, its origins are traced to reformers like Peter Damian who called on monks to renounce riches and emulate the simple life of the Apostles. As much came to be expected of both secular clergy and laity, so a variety of experiments in religious living were tried. Not all proved acceptable to the clerical authorities, but attempts to rein in religious enthusiasms met with resistance. The longest chapter of the book discusses the ‘Cathar’ movement which probably began as just another proposal about how to lead a perfect life. Disapproval drove Cathars to defend their position intellectually by adopting dualist beliefs from the Bogomils of the eastern Church and thus presenting themselves as the true successors of the apostolic age. It is mainly from controversial literature and formal enquiries into heresy that the evidence for this study is derived. The Waldensian movement, though similarly inspired by apostolic example, resisted the appeal of dualism, but forfeited its early episcopal support by insisting that even unordained Christians had a duty to preach the Gospel. Joachim of Fiore, to whose theories and prophecies the next longest chapter is devoted, did not himself face persecution, but the use made of his writings by the Spiritual Franciscans (and those that they in turn influenced, like Arnald of Villanova and Dante) in their views about the Church's shortcomings explains his relevance here. Because this is a theological Habilitationschrift, the author is understandably most interested in the internal logic of the various theological positions. Nevertheless religious innovators were not immune to the influence of circumstances and events. There is nothing here to show why dissidents flourished most in Italy and southern France, nothing about how the papacy was saddled with responsibility for repressing heretics. Its renewed quarrel with the empire, especially under Frederick ii, could not fail to affect reformers who held the Church's relations with the empire after the conversion of Constantine and his alleged Donation to the papacy responsible for compromising the apostolic character of the early Church. The author acknowledges that the movements discussed were not the only ones to express dissatisfaction with the way the medieval Church had developed, but within his chosen terms of reference he demonstrates the efforts of some men, even if they lacked any real historical sense, to rediscover the original sources of Christian inspiration in the life and work of Jesus and his disciples.