Cast your mind back to when Bill Clinton was president, Tony Blair and Vladimir Putin werefresh-faced new leaders and tweeting was strictly for the birds.

That was when technology stocks, as measured by the S&P 500 tech index, last traded at their current levels.

The horrendous decline in share prices that followed the peak in 2000 was the first financial calamity of this millennium.

The dotcom crash had much less impact on the broader economy than the mortgage and banking crisis of 2007-08.

Nevertheless, the tech revival has caused some twitchiness among investors.

Might history be repeating itself?

In the intervening years the world, and the tech industry, have changed a lot.

In the late 1990s enthusiasm for tech shares was so great that the sector’s market value rose far faster than its earnings.

The gap is nothing like as great today.

Back then, leading firms like Microsoft and Oracle were valued at more than 20 times their annual revenues, let alone earnings.

This time around, with the exception of Facebook, price-to-revenue ratios are much lessstretched.

What boosted tech businesses in the late 1990s was that everyone was discovering the internet at the same time.

Both companies and consumers were buying computers and associated items like modems.

That led to rapid revenue growth.

But the sudden enthusiasm for tech was also its greatest weakness; every college graduate seemed to have a plan to start a dotcom company.

The market became overcrowded.

Investors struggled to tell the long-term winners from the losers.