The June 2013 Supreme Court decision United States v. Windsor, overturning a key provision of the Defense of Marriage Act (DOMA), is of historic importance for same-sex married couples in the United States and is undoubtedly a major legal victory for the American LGBT rights movement. With Section 3 of DOMA struck down, we have seen rapid change in the legal treatment of married same-sex couples, including under immigration law. In this (somewhat academic and lawyerly) post I look at how DOMA came to be, examine the Windsor decision in detail, and consider its broader impact. In a separate future post my colleague Liz Brettschneider will answer some practical questions for couples whose immigration cases are affected by the Windsor decision.