The practice of ribā ("usury" or "interest" in Arabic) is forbidden by Islam. This prohibition is inscribed in Islamic Law originating during the Middle Ages in The Arab Peninsula. It is at the core of what is called "Islamic finance", having had a remarkable expansion in the second part of the twentieth century. This article focuses on the origin of ribā prohibition, the problems facing Islamic finance at the eve of the third millennium and the development prospects of Islamic financial institutions.