Inspired by the later medieval development in logic, especially theories on the properties of terms, Ockham’s modal logic is an innovative expansion of Aristotelian modal logic. Ockham’s treatment of modal logic is evolved systematically on the ground of the medieval distinction between two readings of modal propositions, that is, the reading in the divided and in the composite sense, which can be compared to the de re and de dicto reading in modern modal logic. The result is a comprehensive theory of propositional modal logic and syllogistics. In addition to Aristotle’s modal term logic, Ockham works out syntactic rules for inferences of modal sentences in the composite sense and offers a framework for propositional modal logic. In this chapter, I outline Ockham’s modal logic by describing the related texts, semantics for modalities, the linguistic and logical structure of modal sentences, their truth conditions, propositional modal logic, and modal syllogistics in Ockham.