What is ‘radical Pietism’? The term has been established in German ecclesiastical historiography ever since the publication of the second volume of Albrecht Ritschl's history of Pietism in 1884. It usually denotes those individuals and groups who to varying degrees rejected both the institutions and the teachings of the established Lutheran Church in the Holy Roman Empire in the seventeenth and eighteenth century. Over the last fifty years or so Hans Schneider has illuminated the extreme diversity of this tendency. On the other hand scholars such as Hartmut Lehmann have sought to extend the scope of what is meant by Pietism itself, and have been inclined to include in it virtually every renewal movement in German Protestantism from the early seventeenth century to the early twenty-first. These positions were enshrined in the monumental four-volume Geschichte des Pietismus edited by Martin Brecht, Hartmut Lehmann and Ulrich Gäbler (Göttingen 1993–2004). As this new volume of essays edited by Wolfgang Breul, Marcus Meier and Lothar Vogel demonstrates, they are now being subject to careful scrutiny and lively debate. The volume comprises papers given at a Marburg conference in 2007 to mark Hans Schneider's retirement. It unites further reflections by Martin Brecht and Hartmut Lehmann with critical remarks by Johannes Wallmann, who continues to plead for a stricter definition of Pietism, for the primacy of Spener as the originator of the movement and for a distinction between Pietists who remained within the Church and those who abandoned it. Twenty-one fascinating case studies of all manner of ‘radical’ Pietists and ‘radical’ ideas illustrate the difficulty of reaching any clear definition of what exactly is meant by the label. Included are both Lutherans and Reformed (Calvinists), those who flourished within an established church framework and those who either left it or were expelled, those who broadly accepted established teachings and those who rejected mainstream theology, those who preached abstinence and celibacy as well as those who engaged in (often communal) sexual practices that scandalised society and led to their persecution. As usual in the German context, what was ‘radical’ in one territory could often be accommodated within traditional structures in another. Finally, Hans Schneider reflects on his own scholarly development and research agenda. He now doubts whether the previously accepted defining characteristics of Pietism – the formation of conventicles, the espousal of a new eschatology, the insistence on tota scriptura rather than simply sola scriptura – are really sufficient to embrace the full spectrum of Pietism. He attaches particular importance to recent research that has identified Johann Jakob Schütz, rather than Spener, as the real founder of Pietism, the man who translated early seventeenth-century piety and renewal into the idiom that later became known as Pietism. He has come to share the unease felt by many about the term ‘radical’ and prefers ‘heterodox’ and ‘separatist’. Above all, Schneider underlines how much we simply do not yet know about the literature of Pietism, its development in the minor German courts (especially those of the imperial counts), its filiations, networks and ramifications, both within the Holy Roman Empire and internationally. For anyone who thought that the four-volume Geschichte des Pietismus might be the last word, this stimulating volume of essays will be essential reading.