They face arrest, deportation, and death threats. Their work is so sensitive that few are prepared to talk about it.
But for this programme - our religious affairs correspondent, Christopher Landau - has persuaded some of them to speak: the missionaries who carry the gospel to countries which are hostile to Christian mission.
Countries where evangelising may put them, and those they lead to Christ, in mortal danger. Hear their stories, and find out about the huge ethical dilemmas they face.
And discover how their work looks from a Muslim perspective.