This collection of primary sources documenting heresy from early Christian times to the fifteenth century is invaluable for those who are interested in medieval heresy. Peters' introductions are well written, easy to understand, and good analysis and summation of the historiography of the subject. The theme of the book is easy to follow, as it goes chronologically. The pieces Peters selected are (mostly) relevant and worth reading, though, as he notes rather sadly, they are nearly all written by members of the Church, so it is difficult to parse out what might actually have been true and what was common rote on heresiarchs. That said, there are some pieces here that I had never read but that would greatly aid anyone looking at medieval heresy. A great collection, well edited and well selected.