Roman and canon law proscriptions on heresy have long been acknowledged as having had a pivotal influence on the development of secular attitudes towards heresy in the central Middle Ages. As is to be expected of any volume appearing in the MGH Studien und Texte series, Sascha Ragg's meticulously researched and documented study is thus a welcome exploration of the important questions of how secular measures and legislation against heresy were developed and implemented throughout Europe under the influence of Roman and canon law during the high Middle Ages. Ragg adopts a longue durée approach, dividing the study into three parts. The first part addresses anti-heretical measures in late antiquity and the earlier Middle Ages, focusing both on the impact of Roman law – in particular the influence of ideas about public crimes via Justinian and the Theodosian Code among others – and also on the increasing connections of heresy with witchcraft in the early Middle Ages as codified in various ‘barbarian’ law codes and Carolingian capitulary texts. Part ii turns to heresy in the high Middle Ages. Here Ragg briefly addresses the increasingly oppressive legal measures adopted in the eleventh and the first part of the twelfth centuries, the justification of the use of repressive force by Gratian, as well as the increasing systematisation of anti-heretical measures in the thirteenth and fourteenth centuries under Innocent iii, Gregory ix, Innocent iv and the Decretists and Decretalists. These two sections form the background to part iii, which is the core of the study. Here Ragg turns to anti-heretical measures and their political contexts chiefly, but not exclusively, in the twelfth and thirteenth centuries through a series of case studies on Italy, Germany, France, England and the Iberian peninsula. Here Ragg carefully shows the extent to which regional differences, especially with the rise of Catharism and the Waldensians, resulted in more intense measures at a variety of levels in Italy, France and the empire than were required in England or Aragon. The volume also includes a conclusion, an extremely detailed bibliography and index. This is an important work that goes far beyond a synthesis. The quite phenomenal range of sources, to say nothing of critical analysis, that informs this study will make it an indispensable study for anyone approaching the prosecution of heresy in the central Middles Ages, its legal sources and intellectual underpinnings, and perhaps especially the influence and transmission of canonical and Roman law texts in western Europe during this period.