An author who has taken twenty years to complete her manuscript will have incurred many intellectual debts along the way, and the list that follows records my gratitude to those who have helped and inspired me. At every stage, I have been fortunate to work in well-stocked libraries with friendly, knowledgeable librarians. In Oxford, I would like to thank in particular the staff of the Modern Languages Faculty Library (especially Roger Shilcock), the staff of the Taylorian Library (particularly Jill Hughes, David Thomas, and John Wainwright), and the staff of the Bodleian Library. In Spain I was most grateful for the assistance of librarians in the Archivo Capitular, Seville, the Biblioteca Menéndez y Pelayo, Santander, and the Biblioteca de Catalunya, Barcelona. It was also a pleasure to examine two Alfonsine manuscripts in the Vatican Library. Most codices copied for Alfonso are now in Madrid or in the palace and monastery founded by Philip II at the nearby town of El Escorial. I am grateful to Cristina Guillén, rare books librarian at the Biblioteca Nacional, and Elena Luxán, of the Museo del Libro (now the Museo de la Biblioteca Nacional), for allowing me to study some of the library's most precious manuscripts. Manuel Sánchez Mariana and his small team at the Complutensian University library, the Biblioteca Histórica Marqués de Valdecilla, kindly brought me the Alfonsine Libros del saber de astrología to examine on numerous mornings. I would also like to extend my warmest thanks to the director of the Real Biblioteca del Monasterio de El Escorial, Padre José Luis del Valle Merino, and to his colleague Padre Teodoro † for welcoming me and supporting me during my many visits to study the Alfonsine treasures in their collection. The origin of this book is a doctoral thesis supervised by Stephen Parkinson and submitted to the Faculty of Medieval and Modern Languages, Oxford, in Hilary Term 1999. The gathering of evidence for the arguments presented here would not have been possible without the generous support of the British Academy (at both doctoral and post-doctoral level), the Trustees of the Lady Allen Scholarship, the Trustees of the de Osma Studentship, the Curators of the Taylor Institution, and the fellows of The Queen's College, Oxford. I am extremely grateful for their largesse, without which I could not have carried out the crucial task of repeatedly examining the manuscripts.