NSA 'tracking' millions of mobile phones
5 December 2013 Last updated at 19:51 GMT
Almost five billion mobile phone location records are logged by the NSA every day, according to the Washington Post.
Citing documents leaked by the former intelligence analyst Edward Snowden, the newspaper says the information can be used to track people's movements and chart their relationships.
The BBC's technology reporter Jane Wakefield spoke to Jon Sopel about how this affects people.