Securities law is an underappreciated part of a regulatory system that checks the power of big business. Corporate power has three essential elements: market power, managerial power, and allocative power. Antitrust addresses abuses of market power, corporate law limits managerial power, and securities regulation strives to ensure that only worthy companies gain the power to allocate societal resources. This Article shows how the problem of big business has consistently been a concern of federal securities law. In the early part of the twentieth century, both social reformers and economists viewed disclosure as a way of revealing the inefficiency of the trusts. In the modern era, securities law has become an important way of regulating large companies as market valuation has become the primary measure of corporate size. Unlike populist antitrust law, securities regulation accepts bigness and, in some ways, even encourages it. Viewing securities law through the lens of corporate power suggests that the very largest public companies should be subject to heightened disclosure requirements.