Facebook founder Mark Zuckerberg is set to defend Oculus in court today in the lawsuit with id Software owner ZeniMax over the development of virtual reality technology.

Zenimax argued its early innovations in VR were unlawfully copied when Oculus built its own headset, the Rift. Oculus was bought by Facebook in 2014, and is seen as a major part of the firm’s strategy over the next 10 years.

According to the BBC, Zuckerberg will argue that “ZeniMax was slow to appreciate the potential of VR–and that it was Oculus’ own work that made it the valuable technology it is today.”

An Oculus spokesperson told the BBC, “We’re eager to present our case in court.”

“Oculus and its founders have invested a wealth of time and money in VR because we believe it can fundamentally transform the way people interact and communicate.

“We’re disappointed that another company is using wasteful litigation to attempt to take credit for technology that it did not have the vision, expertise, or patience to build.”

Central to the lawsuit is legendary video game programmer John Carmack, who had worked for ZeniMax from 2009, when the corporation acquired his studio Id Software for $405 million. In November 2013, he resigned to join Oculus full-time.

But for several months before his departure from Zenimax, Carmack was effectively working for both companies. A year prior to this, he was also helping build a virtual reality version of Doom 3 for Oculus VR.

ZeniMax accuses Carmack of sharing its intellectual property with Oculus while he was working at id Software and claims he took it with him when he left.

“He never returned those files or all copies of them after his employment with ZeniMax was terminated,” states the lawsuit (via Game Informer). Zenimax lawyer Tony Sammi has said the case is “one of the biggest technology heists ever.”

In a statement, ZeniMax said it and id Software “welcome the opportunity to present substantial evidence” in court.

“That evidence includes the theft of trade secrets and highly confidential information, including computer code. ZeniMax will also present evidence of the Defendants’ intentional destruction of evidence to cover up their wrongdoing. ZeniMax and id Software are the visionary developers of breakthrough VR technology and look forward to the vindication of our claims.”

According to a report at OculusVR – which is sitting in on proceedings – Carmack was questioned last week about his knowledge pertaining to a MacBook which had not been wiped, to which he replied: “I am not a Mac user unless under duress.” Based on that report it sounds like the proceedings are fraught with tension.

ZeniMax is seeking $2 billion in damage, which matches the value that Facebook paid for Oculus in 2014.

Zuckerberg, Luckey, Iribe, and other key Oculus employees are scheduled to testify during the remainder of the three-week trial. They will be asked about how Facebook came to acquire Oculus, the details of how the Oculus Rift headset was invented, and whether Carmack violated his contract with Zenimax. Prior to these testimonies, most of the case’s proceedings have been kept secret and under seal by the court.