The institution of slavery is as old as civilization. Many nations and empires were built by the muscles of slaves.

But what kinds of people were enslaved, and why? In ancient civilizations, slaves were usually war captives. The victors in battle might enslave the losers rather than killing them. Over time, people have found other reasons to justify slavery. Slaves were usually considered somehow different than their owners. They might belong to a different race, religion, nationality, or ethnic background. By focusing on such differences, slave owners felt they could deny basic human rights to their slaves.

And despite many efforts to end slavery, it still exists today. Some 27 million people worldwide are enslaved or work as forced laborers. That's more people than at any other point in the history of the world.

Who was enslaved? What rights did slaves have? How could slaves gain their freedom? The answers are different for every society with slaves. Click on any of the highlighted areas of the map to explore a few of the many slave-based societies in history.