The Department of Justice (DOJ) recently concluded a three-week trial focused on allegations that Google has established illegal monopolies within the ad tech market. Central to Google's defense is the assertion that the DOJ's understanding of the online advertising landscape is flawed. Google contends that the government is mischaracterizing the market by failing to recognize the significant competition it faces from social media platforms. The DOJ has defined the relevant market as comprising open web display ads, which include the ad boxes and banners seen on various websites. It identifies three key components: publisher ad servers, advertiser ad networks, and ad exchanges. According to the DOJ, Google holds a dominant position in this market, particularly through its publisher ad server, DoubleClick For Publishers (DFP), which it claims has a 90% market share in the U.S. The DOJ argues that Google's dominance is partly due to its practice of tying its products together, compelling publishers to use DFP to access Google's extensive advertiser base. In contrast, Google argues that the market should be viewed as a single entity where both buyers and sellers of digital advertising interact. The company claims that its integrated ad tech solutions provide benefits to customers, making it more efficient and cost-effective compared to using multiple separate products. Google’s expert witness, economist Mark Israel, emphasized that the digital ad industry is fundamentally about making connections between buyers and sellers, and that Google's offerings enhance this process rather than create monopolistic harm. A significant aspect of Google's defense hinges on the Supreme Court case Ohio v. American Express, which dealt with two-sided markets. Google aims to demonstrate that the ad tech market operates similarly, suggesting that the DOJ must prove that Google's actions harm both publishers and advertisers. This presents a challenge for the DOJ, as actions that benefit one side may not necessarily be detrimental to the other. The DOJ has also attempted to delineate the market by asserting that open web display ads are distinct from other forms of advertising, such as those on social media or video platforms. Google counters this by arguing that advertisers prioritize return on investment and will shift their budgets to platforms that yield better results, regardless of the specific format of the ads. Internal documents from Google reveal that the company closely monitors competitors that the DOJ has excluded from its market definition, including social media giants like TikTok and Facebook, which Google views as significant threats. Google argues that the presence of these competitors serves as a check on its market power. The trial has seen intense cross-examination, with DOJ counsel challenging the credibility of Google's expert witnesses and their claims about competition. The DOJ has illustrated that even as advertisers may diversify their spending, publishers still rely heavily on display ads, which creates a complex dynamic in the market. Judge Leonie Brinkema has presided over the trial, maintaining a focused approach while seeking clarity on the market definitions at play. As the trial progresses, the judge has indicated that the definition of the market is crucial to the case, and she will consider the entirety of the evidence presented before making her ruling. The closing arguments are set for November 25th, where both sides will have the opportunity to further articulate their positions on the market definitions and the implications for competition in the ad tech industry.