The UK faces a range of established and emerging threats that challenge our national security.
These include cyber attacks, international and domestic terrorism, organised crime, and the spread of nuclear, biological and chemical weapons.
The cyber threat
'Cyber' is the term used to describe the electronic medium of digital networks - including the Internet - that support infrastructure and services. Every day, UK institutions, businesses and individuals rely on the availability of these systems.
Terrorists, 'hacktivist' groups, foreign states and criminals use cyber for malicious purposes. They use computer networks to steal sensitive data (such as intellectual property and research and development projects) and to deploy malicious software which disrupts and damages the cyber infrastructure. This can range from taking a website offline, to manipulating industrial control systems.
The growing threat
As we become more reliant on the Internet in our everyday lives, the threat from cyber will only increase. GCHQ indentifies cyber espionage activity targeting UK industry and individuals, and gathers intelligence to better understand new and emerging threats.
GCHQ also strives to safeguard current systems, communications and electronic data, and provides security advice to the UK's Critical National Infrastructure, such as electricity networks, telecoms and transport.
The threat from terrorism
The terrorist threat we now face comes from a range of international and domestic sources. This includes the core Al Qaida network and its allies, and more recently smaller groups or individuals who are inspired (but not directed) by terrorist organisations.
GCHQ is one of the agencies that contributes to the make-up of the Joint Terrorism Analysis Centre (JTAC), which was formed in June 2003 to provide advice to the UK government and major industries on terrorist threats. GCHQ uses sophisticated technology to provide intelligence that helps inform national security, military operations and law enforcement activities.
The terrorist threat can take a number of forms, and may include explosive devices, firearms, kidnapping, infiltration and cyber attacks. GCHQ, alongside MI5 and the Secret Intelligence Service (MI6), plays a crucial role in countering these threats.
The threats from espionage and serious crime
As worldwide political and technological environments have evolved, GCHQ's priorities have changed to meet them.
Espionage
The threat from espionage did not end with the collapse of Soviet communism in the early 1990s.
In the past, espionage was directed towards obtaining political and military intelligence. Today, our technology-driven world means cyber criminals can target commercial and government-related organisations across fields such as information and communications technology, genetics and defence.
Serious crime
A significant proportion of organised crime is enabled by cyber networks.
In addition to making the UK one of the safest places in the world to do business, GCHQ works closely with the new National Crime Agency to combat crime by sharing intelligence, capabilities and expertise.