A study of the development of the theory of modal syllogistic in the Middle Ages. It traces the theory from the first mediaeval commentators on Aristotle's "Prior Analytics" to the end of the Middle Ages. In the book, several texts are analyzed and the works of philosophers like Robert Kilwardby, Albert the Great, Richard of Campsall, William of Ockham, John Buridan, Pseudo-Scotus, Albert of Saxony, Marsilius of Inghen and Jodocus Trutfetter are studied. These author's views on modal syllogistics are shown to comprise important insights clarifying central issues with implications for mediaeval philosophy in general.