What is modern slavery and who are the victims?
Theresa May has described modern slavery as "one of the greatest human rights abuses of our time" as she prepared to meet victims of the crime in Nigeria.
Q: What is modern slavery?
A: According to campaign group the Walk Free Foundation, it refers to situations where one person has taken away another person's freedom so that they can be exploited.
It includes human trafficking, slavery and slavery-like practices such as servitude, forced labour, forced or servile marriage, the sale and exploitation of children, and debt bondage.
Q: Where are the victims from?
A: They are often brought to the UK and other wealthy nations by human traffickers to be exploited in the sex trade, or used for forced labour, domestic servitude or even organ harvesting.
But they can also be UK residents who are exploited because of their vulnerability.
Q: How widespread is it?
A: According to some estimates, modern slavery is the second largest criminal industry in the world after drug trafficking.
Q: And in the UK?
The Home Office estimates there are 13,000 victims and survivors of modern slavery in the UK. Of these, 55% are female and 35% are trafficked for sexual exploitation.
But some experts believe the true figure may be even higher.
Q: So is anyone being caught and prosecuted?
A: Prosecutions in the UK are up by more than a quarter.
The Crown Prosecution Service received 355 referrals from police and other agencies in 2017/18 and charged 239 suspects - a 27% rise compared with 2016/17.
Q: What sort of crimes lie behind the figures?
A: In one case, a victim was kept in domestic servitude for more than 20 years after being brought to the UK from Nigeria.
Obstetrician Emmanuel Edet, 61, and midwife Antan Edet, 58, who kept Ofonime Sunday Inuk as a "houseboy" in London, were jailed for six years in 2015.
Last year, members of a traveller family were jailed for running a modern slavery ring which kept one of its captives in "truly shocking" conditions for decades.
Operating from sites in Lincolnshire, members of the Rooney clan targeted victims who were homeless, had learning disabilities or complex drug and alcohol issues.