In the concluding part of Who's Watching You? Richard Bilton speaks to leading figures from the shadowy world of secret intelligence to find out how effective it is in tackling serious crime and terrorism.

Intelligence insiders reveal the difficult task they face with an ever-evolving threat and a constant arms race between the state and criminals over surveillance technology.

And Richard talks to former insiders who question the government's argument that if you have nothing to hide you have nothing to fear and who say that is an argument for total surveillance and a total security state.

For more information about surveillance visit the Who's Watching Youwebsite.

Who's Watching You begins concludes Monday 25 May, 2009 at 2100 BST on BBC Two. Or catch up on iPlayer afterwards.