Although excessive consumption of processed food is considered a risk factor for obesity, the caloric contribution of processed foods to US diet has not been accurately assessed, particularly for vulnerable populations. Further, conclusions about the nutritional quality of processed foods are discrepant. Using food and beverage purchases of households (n=157,142) participating in the 2000-2012 Homescan longitudinal panel, this research aimed to develop an innovative approach for defining processed foods and to determine the nutritional role of these products in US diet. In Aim 1, we developed a multidimensional classification system with explicitly defined criteria to categorize foods and beverages by level of processing and separately by level of convenience. We classified >1.2 million items using product-level information and ingredient lists. We further evaluated 13-year trends in the caloric contribution of processed and convenience foods to purchases. We found unshifting dominance of ultra-processed and ready-to-eat (RTE) foods as major calorie contributors to US diet. Aim 2 determined the longitudinal association of sociodemographic and economic household characteristics with processed and convenience food purchases. Less education and lower income were associated with higher ultra-processed food purchases, with associations strengthening across time. Non-Hispanic black households had higher ultra-processed beverage purchases yet lower ultra-processed food purchases compared to non-Hispanic whites. Non-Hispanic black and Hispanic race/ethnicities and lower education were associated with lower RTE convenience food purchases, suggesting a greater role of cooking. In Aim 3, we compared nutrient densities of foods across categories of processing and convenience. The adjusted proportion of household-level food purchases exceeding recommended maximums for saturated fat, sugar, and sodium densities was significantly higher for ultra-processed and RTE food purchases compared to purchases of less-processed foods or foods requiring cooking. In conclusion, our study suggests that ultra-processed and RTE convenience foods and beverages dominate diets in the US with remarkable stability and may contribute to sociodemographic disparities in diet. Our findings of higher saturated fat, sugar, and sodium content of ultra-processed and RTE foods support the need for future studies to examine the relationship of these products with diet and health outcomes.