This dissertation studies a unique subset of legal obligations in Islamic law known as “collective duties” (farḍ kifāya) and focuses on juristic writing in the premodern period between the 9th and 14th centuries C.E. Together with the more widely recognized “individual obligations” (farḍ ʿayn), these duties encompass the complete range of mandated behavior in Islamic law. Individual obligations follow a simple pattern: one person is assigned responsibility for performing a particular act and is solely held responsible if they fail to do so. Collective duties are premised on a different concept involving shared responsibility for required acts. They are based on a formula consisting of two clauses, which loosely draws from a Qurʾānic prooftext. The first clause states that as long as some people perform the duty, then the obligation is suspended for everyone else. While everyone initially carries the burden, they are not all required to perform. However, the second clause adds an important warning: if no one performs the duty, then everyone is held accountable. This study explores the juristic discourse on collective duties in order to better understand how they function, what purpose they serve and why they might have been created. As premodern jurists explored the implications of collective duties as a whole, they developed the theoretical outlines of a kifāya-doctrine, one that asked questions of whether collective duties were preferred to individual obligations, who in the collective was required to perform and when an obligation was suspended. Beyond the general doctrine, the dissertation also examines legal rules developed for three specific collective duties: jihād, funerary rites and duties to rescue. The discourse on these duties demonstrates how jurists not only provided practical guidance for performance of the obligation, but also thought more broadly about the theoretical implications for law. In the process, they began to determine who belongs in the moral community, defined a robust role for the state in law’s implementation and speculated on what should constitute ethical behavior. As a result, they made clear that the normative universe of obligation is essential to understanding the Islamic legal tradition.