The authors of this study identified measures of emergency department (ED) and hospital workflow that would be of value in understanding, monitoring, and managing crowding. A national group of 74 experts developed 113 potential measures using a conceptual model of ED crowding that segmented the measures into input, throughput, and output categories. Ten investigators then used group consensus methods to revise and consolidate them into a refined set of 30 measures that were rated by all 74 experts.