This dissertation explores the development of the concept of custom in the Islamic legal tradition. Within this tradition the concept has been associated with the two terms of `urf and `ādah. After a review of modern scholarship on the subject, the dissertation examines the roots of the concept in the founding texts of Islamic law: the Qur'ān and the sunnah of the Prophet. It also explores the theological foundations of the concept in the early debates on causality. The development of the concept until the 5th/11th century is explored within the literature of the two major schools of jurisprudence: the theoretical school and the applied school. In the post 5th/11th century the development of the concept is explored in the light of the wider expansion of legal theory. More particularly, it is explored in the area of sources through the themes of qiyās and istidlál and in the area of legal hermeneutics through the theme of takhs&dotbelow;īs&dotbelow;. With the emergence of disparate genres of jurisprudence such as legal maxims, qawā`id fiqhiyyah, and legal objectives, maqās&dotbelow;id al-sharī`ah, the concept was further consolidated. The last chapter focuses on the role of custom in the area of legal application through the examination of the two genres of legal respnsa, fatāwā, and judicial verdicts, ah&dotbelow;kām. It is argued that in the Islamic legal tradition, legal sources are arranged in a hierarchical order. Within this order, custom can be a source of law as long as it does not conflict with a higher source. Moreover, the role of custom in the Islamic legal tradition is not limited to the area of sources, but it permeates the different stages of the legal process.