The functioning of modern societies requires large flows of materials to satisfy human wants both directly and indirectly; for example, 50 kg per day per American. The nature of these flows determines their impact on the natural environment. We develop and test a comprehensive framework to order materials flows in the US economy, We assess and quantify inputs to the national economy, outputs, foreign trade, and wastes from resource extraction, using mass measures of these flow components. The bulk of materials inputs satisfies demand for energy, construction, and food. Atmospheric emissions and materials embedded in long-lived structures dominate outputs, with smaller contributions from solid wastes and dissipated materials. Trade, accounting for approximately 10% of US materials flows, is dominated by bulk commodities such as fuel, food, and chemicals. Extractive wastes from fuel and nonfuel minerals account for more than double the amount of inputs and mostly remain at the site of generation, Metrics based on a consistent, periodic accounting of physical materials flows can provide a powerful means to assess environmental performance at the national level. Improvements in the collection and organization of the data supporting national material accounts will further their utility.