Legal scholars have long recognized that the media plays a key role in assuring the proper functioning of political and business markets. Yet we have understudied the role of law in assuring effective media scrutiny. This Article develops a theory of law as source. The basic premise is that the law not only regulates what the media can or cannot say, but also facilitates media scrutiny by producing information. Specifically, law enforcement actions, such as litigation or regulatory investigations, extract information on the behavior of powerful players in business or government. Journalists can then translate the information into biting investigative reports and diffuse them widely, thereby shaping players’ reputations and norms. Levels of accountability in society are therefore not simply a function of the effectiveness of the courts as a watchdog or the media as a watchdog, but rather a function of the interactions between the two watchdogs. This Article approaches, from multiple angles, the questions of how and how much the media relies on legal sources. I analyze the content of projects that won investigative reporting prizes in the past two decades; interview forty veteran reporters; scour a reporters‐only database of tip sheets and how‐to manuals; go over syllabi of investigative reporting courses; and synthesize insights from the communication science and economics of information literatures. The triangulation of these different methods produces three sets of insights. First, this Article establishes that legal sources matter: in today’s information environment, court documents, depositions, and regulatory reports are often the most instrumental sources of accountability journalism. Second, the Article identifies how and why legal sources matter: they extract quality information on the (mis)behavior of powerful players in a credible, libel‐proof manner. Finally, recognizing the function of law as source opens up space for rethinking important legal institutions according to how they contribute to information production. In the process, we get to reevaluate timely debates, such as the desirability of one‐sided arbitration clauses, which have been at the center of recent Trump Administration orders and Supreme Court decisions.