The S&P 500 hit an all time high yesterday as it looks set to break the record for the longest-ever bull run.
Unless the index collapses today (22 August) it will have gone 3,453 days without a fall of 20 per cent or more, which would be the longest-ever rally in US history.
Yesterday afternoon the S&P 500 reached 2,873.23, breaking its previous record of 2,872.87 before falling to close up 0.2 per cent at 2,862.96.
During the nine years of this bull run, the S&P 500 has more than quadrupled in value, which would have turned £100 invested in March 2009 into £555 with dividends reinvested.
Laith Khalaf, senior analyst at Hargreaves Lansdown, said: "They say bull markets don’t die of old age, but this one’s giving it a fair go.
"Many question whether this exceptional period for the US stock market is going to end in tears, though the very fact this issue is so widely raised suggests we are not in the throes of the irrational exuberance of the late 1990s."
He added: "Despite the US economy performing well, the valuation of the stock market does give pause for thought. That said, one could have made the same argument for several years and missed out on some handsome returns.
"Trying to time the market is notoriously difficult, and is just as likely to leave you out of pocket as quids in."
On an annualised basis, this run has seen gains of 16.5 per cent, which is less than the 22 per cent historic average for bull markets.
In recent years the S&P 500's returns have been powered by tech companies, including Facebook, Apple, Amazon, Netflix and Google - collectively known as the Faangs.
The New York Stock Exchange's Faang+ index, which includes other tech stocks such as Tesla, has grown by more than 36 per cent since September 2017 while individual companies have seen much greater growth, with Amazon and Netflix seeing their share price rise by about 100 per cent over the past year.