Understanding passive margins-thick accumulations of sediments built above the juncture between continental and oceanic crusts-has far-reaching economic and societal implications. Passive margins underlie the coastal regions of most of the conterminous United States, extending continuously from Texas eastward to Florida and northward to Maine. They hide most of the undiscovered hydrocarbon reserves of the United States, and they are excellent sites for sequestering carbon dioxide. Natural hazards of hurricanes, tsunamis, sea level rise, and rapid subsidence, and concerns related to the United Nations Law of the Sea, also make it imperative to better understand passive margins, and how they form and evolve.