How do countries spy on each other?

Diplomatic relations between America and its allies continue to show strain after revelations of large-scale intelligence gathering on citizens, and the interception of some leaders' phones.

But how do agencies like the National Security Agency gather such large quantities of information?

We speak to those who have worked for intelligence agencies around the world about the realities of eavesdropping and surveillance.

(Photo credit: REUTERS/Kai Pfaffenbach)