A classic historical study that examines the roots, structure, and operational methods of the Medieval Inquisition in Europe. The book traces how ecclesiastical courts evolved into a systematic and powerful system of interrogation. Lea argues that the Inquisition was not a haphazardly created system by the Church, but rather a natural evolution of 13th-century social conditions and intellectual thought, which saw heresy as a real threat. The book details how this institution was formed. The author vividly describes the methods of interrogation, the use of secret witnesses, trials by torture to extract confessions, and the punishments imposed (such as burning at the stake). The text examines how the Inquisition was implemented in various regions of Europe, particularly in France, Spain, and Italy. Lea also highlights the local responses and resistance to the institution by local communities and authorities. The final section examines how the Inquisition was often used for political ends, suppressing religious movements and inciting mass hysteria over accusations of witchcraft and the occult. This work is renowned for its comprehensive research using primary sources and is one of the most fundamental reference texts in the study of Church history and persecution in Europe.