The thesis examines the development of the concept of Ijmāʻ, consensus, in Imāmī-Shīʻī principles of jurisprudence (uṣūl al-fiqh). In the introduction to the thesis there is an analysis of the concept of Ijmāʻ as generally understood in Islamic jurisprudence and a discussion of the approach adopted in the thesis as well as a general outline of the literature involved. The introduction also outlines the background to Imāmī Shīʻī jurisprudence during the time of the presence of the Imāms and the period immediately after the greater occultation of the Twelfth Imām. The main body of the thesis analyses the available major texts written by Imāmī-Shīʻī Uṣūli scholars from the time of al-Shaykh al-Mufid (413/1022) to the end of the nineteenth century with a brief discussion of the views of some recent Imāmī Shīʻī scholars. From the outset a difference of emphasis can be observed between scholars who argue in favour of a major role for reason, such as al-Sharīf al-Murtaḍā (436/1044) and those in favour of a greater reliance on Traditions from the Prophet and the Imāms, which is moderately represented by al-Shaykh al-Ṭūsī (459/1067). The subsequent generations of scholars refined and further defined these concepts. In particular, in opposition to the movement in favour of general adherence to the Traditions, there arose from the proponents of the use of uṣūl al-fiqh a definition which gave much greater scope to the use of reason and the continuing guidance of the Twelfth Imām. The latter proposition reached its final form in Imām-Shīʻī uṣūl al-fiqh at the end of the nineteenth century in Kifayat al-uṣūl by Muḥammad Kāẓim al-Khurāsānī. Finally, there is a brief examination of the work of some recent scholars and a conclusion to the thesis.