This chapter explores the long history of slavery and international law. It notes that the first multinational legal agreements in the modern era were about ending the Atlantic slave trade and then slavery world-wide. The chapter intertwines legal history with modern international law issues, tracing the support for slavery in ancient law, customary international law, canon law, the domestic law of various western nations and modern international law as late as the mid-19th century. It also traces the struggle to end slavery from the 16th to the 19th century, also addressing the international struggle against the African slave trade and later against bondage. The chapter discusses the difficulty of ending slavery and the hypocrisy of some nations in the late 19th and early 20th centuries, as they campaigned against slavery while imposing brutal labor conditions on colonized people in Africa, and at the same time used the claim of ‘ending slavery’ to justify the colonization of Africa. Western nations pledged in the Slavery Convention of 1926 that ‘No one shall be held in slavery or servitude; slavery and the slave trade shall be prohibited in all their forms’. However, shortly after this slavery reappeared, with millions enslaved by the Soviet Union, Japan, and Germany. The 1948 Universal Declaration of Human Rights condemned slavery, incorporating the evolving legal rules of the previous two centuries, and by 2012 it had been formally abolished in all existing nations. The shocking return of slavery as a legally recognized form of property in territories controlled by ISIL (or ISIS) is discussed. Although not recognized by other nations, ISIL has indeed taken on many of the formal attributes of sovereignty, and thus slavery is once again ‘legal’ in at least one jurisdiction.