The concept of ancient slavery encompasses practices so varied across different cultures and periods that the single word threatens to obscure more than it reveals. Slavery in Athens differed from slavery in Rome, which differed from slavery in Mesopotamia, which differed from the New World chattel slavery that shaped modern understandings of the institution.

Athenian slavery was extensive — estimates of 80,000–100,000 enslaved people in 4th century BC Athens, perhaps a third of the total population — but legally and practically different from the plantation slavery of the American South. Enslaved people in Athens could work for wages (returning a portion to their owners), manage businesses, hire out their own labor, accumulate savings, and in some cases purchase their freedom. The enslaved silver miners at Laurion faced conditions that were among the worst in the ancient world; the enslaved secretary or tutor in a wealthy household faced very different conditions.

Roman slavery is documented in extraordinary detail, from philosophical discussions of the slave's humanity (the Stoics' insistence on the slave's equal rational nature) to the practical manuals of agricultural management (Columella, Varro) that recommend treating enslaved agricultural workers with decent care as an economic investment. The slave revolts — the Sicilian slave wars and Spartacus' revolt — reveal the desperation produced by the most brutal end of the spectrum.

The institution's presence in virtually every ancient civilization, and its acceptance by even the most morally serious ancient thinkers (Aristotle's defense of 'natural slavery' is the most notorious example), presents one of the sharpest challenges to any theory of moral progress. The Stoic and Christian critiques of slavery coexisted with their practitioners' ownership of enslaved people for centuries.