It seems hard to imagine now but since about 1500 there was a lot of money to be made buying and selling people as slaves.
Companies would go to Africa and capture men, women and children to be taken in horrific conditions on ships and destined for a cruel life in slavery.
Cities such as Bristol, London and Liverpool grew rich off the trade.
It's thought about 24 million Africans were sold to slave traders.
The slaves were usually sent to work in the sugar cane fields on plantations - giant farms - in the Caribbean, North America and South America where the work was really hard and dangerous. Many were left disabled by work accidents.
Other slaves were used as personal servants in polite society in cities such as London and Edinburgh.