This chapter surveys the metaphysics of the great Shia philosopher and theologian Naṣīr al-Dīn al-Ṭūsī (1201–1274) as it appears in his Ḥall mushkilāt al-Ishārāt, his commentary on Avicenna’s al-Ishārāt wa-l-tanbīhāt. While the chapter treats issues from throughout the commentary, the primary focus is the discussions from nimāṭ 4 and 5. These sections include such central philosophical topics as ontology, causation, and cosmology in which are integrated such key theological doctrines as God’s existence, divine simplicity, God’s causal relation to the cosmos, and the age of the cosmos. A secondary theme of the present chapter is al-Ṭūsī’s response to the commentary of the renowned Ashʿarite theologian Fakhr al-Dīn al-Rāzī (1149–1209) and some of the latter’s more trenchant criticisms of Avicenna’s al-Ishārāt.