The FBI and CIA are agencies that work to protect the USA.
The CIA - Central Intelligence Agency - was set up in 1942 and monitors threats to the US from abroad.
It reports to the US President.
The FBI - Federal Bureau of Investigation - is in charge of intelligence inside the US. And it has the power to enforce the law - a bit like the police.
It has more than 11,500 Special Agents and Osama Bin Laden is on their 10 Most Wanted list.
But unlike in the TV series there isn't really a department called the X-Files.