Here is a breakdown of some of the leading agencies:
DNI: The director of national intelligence is charged with overseeing all the spy agencies, while also "coordinating" counter-terrorism efforts by the Federal Bureau of Investigation and the Department of Homeland Security.
Although the post was designed to provide a strong, single leader to speak for a large bureaucracy and advise the US president, the director lacks authority over the budgets and personnel of the spy agencies, as well as the covert espionage carried out by the CIA.
NCTC: Reporting to the director of national intelligence, the National Counter-Terrorism Center is supposed to "connect the dots," acting as a hub for collecting and analyzing information on possible terror threats.
The NCTC has come in for criticism in the days since the Christmas Day plot last year, with some unnamed officials pointing the finger at the center for failing to piece together clues on the attempt to blow up the aircraft bound for Detroit.
The center manages a list of people suspected of links to extremists, which now has mushroomed to some 500,000 names, and alerts the FBI to the most alarming threats.
The FBI in turn uses the information for a consolidated terrorist watch list. The Federal Aviation Administration draws on the watch list to create a "No Fly List" to prevent suspected terrorists from boarding aircraft.
The CIA and other agencies contribute analysts to the NCTC, who pore over a mountain of data and cables and produce intelligence reports.
The center has struggled to link up separate computer systems and databases that in the past hampered communications among the different agencies.
CIA: Established in 1947, the Central Intelligence Agency long served as the dominant spy service from its headquarters in Langley, Virginia, overseeing the country's other agencies and numerous clandestine plots, including coups in the 1950s and 1960s.
But the CIA's reputation was in tatters after 9/11 and particularly after the debacle over Iraq, where Saddam Hussein's supposed weapons of mass destruction were never found despite the agency's reports.
Congressional reform effectively demoted the CIA, stripping it of its role as presidential adviser and as chief of the other spy agencies.
The CIA, with about 20,000 employees, is believed to carry out drone bombing raids in Pakistan against Al-Qaeda and Taliban militants.
NSA: The National Security Agency, created in 1952 and based at Fort Meade in Maryland, gathers electronic intelligence through a vast eavesdropping operation and a team of analysts schooled in cryptography.
In tapping into the world's email, telephone and fax traffic and databases, the NSA relies on a network of spy satellites overseen by the National Reconnaissance Office, based in Chantilly, Virginia.
The NSA was embroiled in controversy under George W. Bush's administration, which ordered the agency to wiretap some phone calls within the United States without a warrant.
The agency seeks a low public profile and was so secret that the government used to refuse to acknowledge its existence, earning it the nickname "No Such Agency."
DIA: The Defense Intelligence Agency, created in 1961, is one of several spy agencies that fall under the Defense Department's authority, including the NSA.
Led by a three-star general and based inside the Pentagon, the DIA focuses on military intelligence, providing information in support of military planning, operations and weapons programs. With more than 16,000 employees, the DIA oversees the intelligence efforts of each of the branches of the armed services.