Bitcoin explained: How do crypto-currencies work?
An estimated £3-4bn is being laundered via crypto-currencies in Europe every year, the director of Europol has told the BBC.
It comes as Treasury Select Committee member Alison McGovern MP says much speedier regulation is needed.
But what are crypto-currencies? Spencer Kelly explains all.
Who Wants to Be a Bitcoin Millionaire? was first broadcast at 20:30 GMT on Monday 12 February, BBC One.
The programme will be re-broadcast on BBC World News on Saturday 17 and Sunday 18 February 2018.