In a landmark legal battle unfolding in Los Angeles, Mark Zuckerberg is set to take the stand in a trial that scrutinizes whether Meta’s social media platforms are purposefully designed to be addictive and detrimental to young users.

The CEO of Meta will face rigorous questioning on Wednesday from legal representatives of a woman, now 20, known only by her initials KGM. She alleges that her early engagement with social media led to addiction, worsening her depression and suicidal ideations. While TikTok and Snap have already reached settlements, Meta Platforms and YouTube, owned by Google, remain defendants in this case.

Zuckerberg has previously addressed youth safety concerns on Meta’s platforms, both in trials and before Congress, where he also expressed regret to families affected by social media-linked tragedies. However, this trial is notable as it is his first time answering such questions before a jury, with bereaved parents anticipated to fill the limited public seats in the courtroom.

This lawsuit is one of three chosen as bellwether trials, potentially setting a precedent for how numerous related lawsuits against social media firms might proceed in the future.

A spokesperson from Meta has stated that the company firmly rejects the lawsuit’s claims, expressing confidence that the evidence will demonstrate their longstanding dedication to supporting young users.

During his opening remarks, Meta’s lawyer, Paul Schmidt, acknowledged KGM’s mental health issues but challenged the notion that Instagram significantly contributed to them. He referenced medical records indicating a troubled home environment and argued, alongside YouTube’s attorney, that KGM used the platforms as a form of escapism from her mental health challenges.

Zuckerberg’s testimony comes a week after that of Adam Mosseri, the head of Meta’s Instagram, who said in the courtroom that he disagrees with the idea that people can be clinically addicted to social media platforms. Mosseri maintained that Instagram works hard to protect young people using the service, and said it’s “not good for the company, over the long run, to make decisions that profit for us but are poor for people’s well-being.”

Much of Mosseri’s questioning from the plaintiff’s lawyer, Mark Lanier, centered on cosmetic filters on Instagram that changed people’s appearance – a topic that Lanier is sure to revisit with Zuckerberg. He is also expected to face questions about Instagram’s algorithm, the infinite nature of Meta’ feeds and other features the plaintiffs argue are designed to get users hooked.

Meta is also facing a separate trial in New Mexico that began last week.