The US Federal Reserve has taken the historic decision to start unwinding its huge $4.2 trillion (£3.1 trillion) portfolio of bonds built up through its stimulus efforts following the financial crisis.
After buying up US treasuries and mortgage-backed securities as part of 'quantitative easing', the US central bank's balance sheet has swelled to $4.5 trillion, around four times its size before the financial crisis.
Now the Federal Reserve will start to unwind those holdings, as it looks to move monetary policy away from an emergency footing.
Rather than simply selling the bonds it owns, the Fed will cut the amount of proceeds from bonds that mature it reinvests into other bonds.
From next month, it will reinvest $10 billion each month less into bonds, increasing the 'run-off' amount by $10 billion each three months until it reaches $50 billion.
The Federal Reserve's move has been well flagged, with markets reacting calmly to the news.
'The significance of the announcement, whilst expected, should not be under appreciated,' said Adrian Lowcock, investment director at Architas.
'This is a once-in-a-generation change. Over the past decade we have been used to central banks buying bonds each and every day. That has now changed and when the programme gets up to speed the Fed will be reducing its balance sheet by $600 billion a year.'
The Fed has been at pains to avoid a repeat of 2013's 'taper tantrum', when its announcement it would start reducing the amount of bonds it bought sparked a sell-off in the financial markets.
Fed chair Janet Yellen has said she wants the balance sheet reduction to be as boring as 'watching paint dry', and its slow pace seems designed to calm markets.
Under the Fed's current plans, it would take until early 2024 for the central bank to unwind its bond holdings, according to Russ Mould, investment director at AJ Bell.
Alongside pulling the trigger on the unwinding of quantitative easing, the Fed kept interest rates on hold, but the tone of its announcement prompted investors to price in a higher likelihood of a December rate rise.
'The basic message here is US economic performance has been good,' said yellen at a press conference following the Fed's two-day meeting.