Modern slavery is different from historical slavery because although today people may live in awful conditions and be forced to do things, they still have human rights.
So for example if a young girl was kidnapped today and made to work as a servant, but then managed to escape, then the police would protect her.
In the old days, if she was a slave and escaped she would be returned to her master by the police and probably whipped for being a disobedient slave.
One in six people who are forced to work are victims of what's called human trafficking, a business that makes huge amounts of money for those who run it.
It's thought there are about 5 or 6 million children who are forced to work. Some of them are born into slavery, others are sold by their parents or stolen - they work in farming, factories and for richer people.