There were a number of modal systems developed and debated among the Arabic logicians. The material devoted to the topic is too voluminous for anything more than a sketchy account of one line of development and debate. I follow a few aspects of Avicenna's syllogistic through its treatment in the thirteenth century, and its transformation into a compact body of doctrine taught in the madrasa. With regret, in this redaction of the entry I omit mention of Alfarabi and Averroes, not because they are not important, but because, first, Alfarabi's most important treatment of the syllogistic is lost and, secondly, Averroes stands outside the Avicennan tradition of logic.