This thesis is about Ibn Taymiyya's thinking on jihād and baghy. It aims to make an important contribution to the study of early Islamic political thought. It considers how the terms jihād and baghy have developed and been expanded from the structure established by the Qur'an and hadith. It also examines the relationship between jihād and baghy in Islamic law and reveals the pivotal role of the imām in politics. The main focus of this study is Ibn Taymiyya's thinking on jihād and the fatwās that resulted from it, using hitherto overlooked printed materials. It also seeks to explain why Ibn Taymiyya upheld jihād against the Mongols, the Franks and the heretic Shī'a. The thesis is divided into four chapters and structured as follows. The first chapter deals with the life of Ibn Taymiyya. This chapter moves beyond conventional biography to relate the story of Ibn Taymiyya's life to the main events that occurred during the Mongols' incursion. The second chapter identifies what Meccan and Medinan Qur'anic texts say about jihād, and examines whether jihād is a mechanism of self defense or an act of aggression; it also explains the relationship between jihād and the establishment of dār al-Islām, dār al-ḥarb and dār al-'ahd. The third chapter considers Ibn Taymiyya's view on jihād. The fourth chapter analyses Ibn Taymiyya's view on baghy, and aims to arrive at a clearer picture of the relationship between Ibn Taymiyya's concepts of jihād and baghy.