The Federal Reserve will pump $600bn (£373bn) into the US economy by the end of June next year to try and boost the fragile recovery.
This stimulus, which equates to $75bn a month, is slightly higher than economists had expected.
It has been called QE2, after the Fed pumped $1.75trn into the economy during the downturn in its first round of QE.
The US economy grew by an annual rate of 2% between July and September, which is not enough to reduce high unemployment and some see QE as the last chance to get the US economy back on track.
Interest rates are already close to zero, which means the Fed cannot reduce rates any further in order to boost demand- the more traditional policy used by central banks to stimulate growth.
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