Citizens United v. FEC, 558 U.S. 310 (2010), held 5-4 that the First Amendment bars the government from prohibiting independent political expenditures by corporations and unions, overruling Austin v. Michigan Chamber of Commerce and part of McConnell v. FEC. This UpLaw library article traces the doctrine from Buckley v. Valeo through Bellotti, MCFL, Austin, McConnell, and Wisconsin Right to Life, then explains the decision itself, the Stevens dissent, and how the D.C. Circuit's SpeechNow.org ruling and two 2010 FEC advisory opinions actually built the Super PAC. It untangles the difference between contributions and expenditures, shows why dark money flows through 501(c)(4) social-welfare organizations, and follows the quid-pro-quo definition of corruption through McCutcheon v. FEC and FEC v. Cruz. It maps the reform menu—the DISCLOSE Act, tighter coordination rules, small-donor matching, and a constitutional amendment—separating what is available within the decision from what would require changing the constitutional baseline. Worked hypotheticals and cross-references make the architecture concrete for lawyers, business owners, and policymakers alike. The treatment is analytical and nonpartisan, presenting the majority and the dissent on their own terms.