After months of tension, Epic Games is finally getting its day in court with Apple.
In their opening statements, the two companies duelled over Apple's policies and whether its notoriously tight control over its devices constitutes a monopoly.
Epic argued Apple's App Store policies, which require that developers submit apps to Apple for review before they can be sold or given away for free, was unnecessary.
Apple boss Tim Cook will be giving evidence, the first time he's given testimony at a trial.
At stake is the future of the App Store and the amount it charges developers - a wildly lucrative money spinner for the company.
The storm began in August last year, when Epic Games laid a trap for Apple.
Its hit game Fortnite implemented its own in-app payment - bypassing Apple's 30% charges.
Apple promptly kicked Epic Games off the App Store.
But Epic Games was waiting for just that.
It slapped Apple with a 65-page lawsuit - and had even prepared a high production video, a spoof of Apple's iconic 1984 advert for the Apple Mac.
Epic Games has for years claimed the charges imposed on it by Apple are extortionate.
"Epic is not suing for damages," one of Epic's lawyers said. "Epic is suing for change, not just for itself, but for all developers."
Apple pushed back, saying its App Store has helped fuel new multi billion dollar companies.
The fiery speeches from both companies kicked off what may be one of the most important antitrust lawsuits in years. Apple's iPhone is one of the most popular consumer products ever made, with more than 1 billion of them being used today.
In a Senate hearing two weeks ago, Apple's Chief Compliance Officer Kyle Andeer was grilled by lawmakers. Politicians of all stripes - usually so divided on policy - were united in their attacks on Apple.
Democrats Amy Klobuchar and Richard Blumenthal and Republicans Mike Lee and Josh Hawley all took up similar lines of questioning.
Senator Klobuchar said that Apple's App Store was a "literal monopoly".
On Friday the European Union announced that it was charging Apple for its behaviour on the App Store.
Although Epic says allowing competing App Stores and payment processing on iPhones would lower prices and foster competition, Apple said it would need to find new ways to recoup the development and operation costs for its App Store.
The companies are arguing before a California judge in what's called a bench trial, meaning there's no jury. This allows the companies to plead their cases without having to educate and then sway a jury, setting up an easier appeals process when a decision is handed down.