The institution of qasāma has intrigued both Muslim jurists and western scholars. The first were puzzled by its violation of essential legal principles, the latter by its apparent pre-Islamic origins. Because of its archaic and irrational character, western scholars assume that the institution was not applied in practice: "[I]t does not appear that this institution functioned much, even in the past, when the penal law of Islam had a certain practical application." However, the evidence of fatwa collections shows that the qasāma was indeed enforced by courts as late as the nineteenth century, and the rules connected with it have now found their way into some modern Islamic criminal codes. The qasāma, it appears, was a living institution in Islamic law and not just theory. In this essay I will try to shed some light on the origins of this institution and its reception into Islamic law. I will attempt to chart the earliest developments of Islamic jurisprudence by analyzing the available hadith material and the statements of the first generation of jurists. In the conclusion I will suggest that my analysis of the material on qasāma corroborates Motzki's and Powers' revision of the chronology of the development of Islamic jurisprudence first put forward by Joseph Schacht in The Origins of Muhammadan Jurisprudence (1950).