Stock markets around the world have started 2018 where they left off in 2017.
On Wall Street, the S&P 500, the Nasdaq and the Dow Jones Industrial Average are all trading at all-time highs.
In Japan, the benchmark Nikkei 225 is at levels last seen at the end of 1991, while elsewhere in Asia the Hang Seng in Hong Kong is just 4% below its all-time high and the Kospi Composite index in South Korea just 2% off its record close.
The ASX in Australia is also at an all-time high.
And in Europe, the Dax in Germany is close to its all-time high, the Cac 40 in France has today hit a new record, as has the FTSE 100 in Britain.
In short, the picture is buoyant almost everywhere. The FTSE World All-Share Index, which rose by 22% last year, has already risen by a further 2% in 2018 while the MSCI World Index, which rose by 20% last year, is also up by almost 2% so far this year.
But what is behind these record-breaking moves – and can the good times continue to roll?
The main reason for the spectacular run has been solid economic growth around the world.
For the first time since the financial crisis, the economies of the US, China, Japan and Europe have enjoyed a period of synchronised growth that, crucially, also looks reasonably sustainable.
That has translated into stronger earnings growth – optimism over which is the key factor that drives share prices higher.
Another factor driving markets, again resulting from greater optimism, is mergers and acquisitions (M&A) activity.
The end of 2017 in particular saw three enormous deals that touch all parts of the globe, with the US health store chain CVS paying $69bn (£50.9bn) for the US health insurer Aetna; Disney agreeing to buy the filmed entertainment and TV assets of 21st century Fox for $66bn (£48.6bn) and the Westfield shopping centre empire being acquired for $25bn by Franco-Dutch rival Unibail-Rodamco.
There’s also an enormous hostile takeover in the works at the moment, with the chipmaker Broadcom seeking to buy its rival Qualcomm for a jaw-dropping $130bn (£95.8bn), while all the signs are that M&A activity will continue to be strong in 2018.
This is particularly the case in the US, where the Trump tax cuts have given companies a lot more money to play with, while Chinese companies continue to be acquisitive everywhere.
In Europe, where M&A in 2017 grew even more strongly than in the US, continued economic growth should also ensure a continued appetite from businesses to buy competitors.
However, with many stock indices already at record levels and valuations looking stretched, it is easy to see why there is a lot of anxiety among investors.
In fairness that has been the case for quite a while, with the bull market in the US shortly to enter its tenth year, making it one of the longest in history.
In the UK, while markets have not enjoyed quite the same meteoric growth – the FTSE 100 actually suffered reverses in each of 2011, 2014 and 2015 – there is also unease at the strength of the rally during the last two years and whether it can be sustained.
This is not all that surprising. Markets, in the jargon, always climb a “wall of worry” and it is perfectly rational for investors to be wary after such a long period of gains.
Mark Dampier, head of research at the stockbroker Hargreaves Lansdown, was saying as long ago as October 2016 that this was the “most hated” bull market he had seen in his near 35 years in the investment industry.
Investors who decided to head for the sidelines at that time have since missed out on 15% gain for the FTSE 100 – even more if dividends are taken into account.
It is not just investors in the UK who have been warily sitting out the rally. The Wall Street Journal reports today that, since 2012, American investors have withdrawn $1tn (£740bn) from mutual funds.
It says the proportion of Americans who own shares has fallen from 62% prior to the global financial crisis to just 54% today.
Intriguingly, fewer companies have been coming to market as well, pointing to possible wariness among the owners of businesses as well.
The last year in which the number of shares in publicly listed US companies actually rose was as long ago as 2010, since when, the number of shares in issue has shrunk.
That partly reflects M&A activity and record amounts of share buy-backs, but also points to company owners preferring to stay private.
Yet the rally, particularly in the US, is clearly in its late stages. After nine years of a bull market, some kind of reversal is inevitable.
Central banks around the world are beginning to tighten monetary policy everywhere, gradually bringing the age of ultra-cheap money to an end, although interest rates will still remain at very low levels, by historic standards, until at least the end of the current decade.
Likewise, valuations do look stretched and will require meaningful improvements in company earnings to remain justified, although respected commentators such as David Kostin, the chief equity strategist at Goldman Sachs, argue that a combination of above-trend growth in the US and elsewhere, coupled with the Trump tax cuts, mean the US bull market will continue in 2018.
Valuations are equally stretched in bond markets and, without quantitative easing to support the latter, they are likely to lurch downwards in sympathy if equity markets take a tumble.
However, you want a really worrying sign, it is this. The renowned British investor Jeremy Grantham, co-founder of US fund manager Grantham Mayo Van Otterloo, has just said there could be spectacular “melt-up” that might push up stock prices by a further 50% at some point during the next six months to two years before the bubble finally bursts.
Mr Grantham, who is famed for getting out before the end of the dot-com boom in 2000 and the global financial crisis in 2008, has long argued that markets are over-valued.
Bull markets tend to end when the final bear turns bullish.