He is one of the highest placed intelligence officials to ever blow the whistle on insider NSA 'knowings.' He made headlines when he resigned in 2001 after 9/11, having worked more than thirty years for the agency. He was a leading code-breaker against the Soviet Union during the Cold war, and was repelled by the United States' massive surveillance programs.
He's had quite the go, starting in 2002 when he let the public know of a system ( 'trailblazer') intended to analyze data carried on communication networks (like the internet). He exposed the agencies eavesdropping program and has faced harassment from the FBI, NSA and more. He has been in and out of the court room ever since he decided to resign and blow the whistle.
These mass surveillance leaks are nothing new, as we've seen by the leaks recent NSA whislte-blower Edward Snowden has provided us over the past few years.