This chapter examines the history of the Islamic legal tradition. The notion of written law was firmly implanted in the world by the time of Muhammad. However, written law had not entirely displaced chthonic law in Arabia, so the law to which many of the people of Muhammad had been loyal was a particular variant of that cosmos-loyal ethic that simply tells people of their way to live. Islamic law represents a highly developed and complex legal tradition. There is an ongoing necessity of justification of Muhammad's revelation as source of law, given the weight of social practice it must support.