(Washington, DC) — The CEOs of four of the world’s biggest tech companies will defend their businesses in an antitrust hearing before Congress today. Amazon’s Jeff Bezos, Facebook’s Mark Zuckerberg, Apple’s Tim Cook and Google’s Sundar Pichai will testify remotely today at a congressional antitrust hearing.The CEOs will reportedly argue that they face fierce competition from each other and from other large tech companies. Bezos has never testified before Congress and is expected to get the most attention.Bezos will claim Amazon occupies a small share of the overall retail market and that Walmart is twice as big. Zuckerberg will defend Facebook’s acquisition of Instagram and WhatsApp, saying the takeover helped both companies to grow. Google has already faced fines from the European Commission’s competition authority over its shopping search tool and Android mobile operating system. In the U.S.,the Department of Justice is investigating Google’s search product and digital advertising marketplace. A lawsuit may be filed if prosecutors determine Google favors its own products or suppresses competing options through its dominance in search and digital advertising.Facebook disclosed a new Federal Trade Commission probe into its business last year, just after the agency closed a privacy probe into the company with a $5 billion fine. Several state attorneys general have launched their own joint antitrust probe into the company. The company has more than 2 billion monthly users, making it the largest social media platformThe estimated wealth of the four companies is around $5 trillion.