This book examines the contributions of Petrus Peckius (1529–1589), a law professor at the University of Leuven, focusing on how he shaped canon law amid the rise of Protestantism, ongoing wars, and the demands of Catholic Reform, particularly the Council of Trent (1545–1563). This research shows that while Peckius largely adhered to established canons and jurisprudence within its commune, he also engaged with reform, introduced methodological innovations, and adapted to contemporary challenges. Based on Peckius’ published works and lecture notes taken by a student in Leuven (1575–1577), we provide insight into his role as both jurist and educator. The emphasis lies on two key issues explored by Peckius: (1) the reparation of churches damaged during the Iconoclastic Fury and (2) the regulation of clerical conduct.