The first volume of the critical edition of the writings of Huldrych Zwingli was published in 1905, and although two wars and many changes of editors have made progress slow, it is now so nearly complete that it has served as the basis for this study. In almost every case the printed text has been compared with the original manuscript or first printed edition, but in general the work of the editors was so correct as to make direct reference superfluous. In particular, the depth and thoroughness of the many contributions of Walther Koehler have been exploited.

The Librarian of the Zentralbibliothek, Zurich, and successive Directors of the Staatsarchiv have allowed a foreign visitor easy access to the mass of materials under their control, and special thanks are due to them and to the librarians of the Bürgerbibliothek at Bern and other Swiss libraries. The original suggestion that I should turn to Swiss history came thirty years ago from Sir Herbert Butterfield. I have profited by conversations with the late Dr Leonhard von Muralt, with the late Dr Fritz Blanke, and with Dr Ulrich Gaebler in Zurich. Professor A. G. Dickens has helped, especially with the section on Luther, and Professor E. G. Rupp has made valuable suggestions. Professor James Atkinson has frequently assisted one who is no theologian on a number of technical points, and Professor F. P. Pickering has advised about many an otherwise unintelligable German word and phrase.