In an unprecedented hearing, four of the biggest names in tech will give evidence to members of the US Congress.
Mark Zuckerberg (Facebook), Sundar Pichai (Alphabet, including Google), Tim Cook (Apple) and Jeff Bezos (Amazon) will all face questions over their business practices and their impact on smaller rivals.
One of the matters concerning the committee is the degree to which three of the tech companies now control the market for online adverts.
Amazon, Facebook and Google jointly account for more than 62% of internet ad spending in the US, according to market research firm eMarketer.
Amazon’s high placing sometimes surprises people, but if you look closely at its online store you can see many of the products are only as prominent as they are because the brands responsible have paid for a “sponsored” position.
However, Google and Facebook still have a much bigger share.
Facebook’s Instagram division alone now accounts for 12% of the market, and is responsible for the fact its parent company is still seeing ad sales growth.And Google is the biggest player of all, although there have been some forecasts it might see its share shrink this year as a result of a fall in travel industry spending caused by the coronavirus.
Apple does sell some online ads – it lets software publishers buy prominent placings in its App Store – but this represents only a small part of its business.