The North Atlantic Treaty Organisation (NATO) has been an intrinsic part of global security for over 65 years.
Formed in the aftermath of World War II, NATO's original goals were to secure peace in Europe, promote cooperation among its members, and to guard their freedom (largely from, at the time, the threat posed by the USSR).
NATO says it has always sought to promote democracy, individual liberty, the rule of law, and peaceful resolution of disputes.
When the alliance was formed in 1949, it had 12 member states - today its membership encompasses 28 countries.
After the fall of the Berlin Wall and subsequent end of Cold War, NATO's usefulness came into question.
But the end of the Cold War ushered in a new era of instability, and NATO's original remit has now expanded.
It now includes working with non-member countries as one of its tasks, as it works towards resolving a number of complex issues.
These include instability on NATO's eastern and southern borders, terrorism, the proliferation of WMDs, cyber attacks, threats to energy supplies, and the security threats posed by global warming.
All 28 members are represented in the North Atlantic Council, and all decisions are taken by unanimous vote.
NATO does not have its own armed forces, but rather co-ordinates each member nation contributing their own forces, which, taken together, are hoped to add up to more than the sum of their parts.