Mass casualty triage is the process of prioritizing multiple victims when resources are not sufficient to treat everyone immediately. No national guideline for mass casualty triage exists in the United States. The lack of a national guideline has resulted in variability in triage processes, tags, and nomenclature. This variability has the potential to inject confusion and miscommunication into the disaster incident, particularly when multiple jurisdictions are involved. The Model Uniform Core Criteria for Mass Casualty Triage were developed to be a national guideline for mass casualty triage to ensure interoperability and standardization when responding to a mass casualty incident. The Core Criteria consist of 4 categories: general considerations, global sorting, lifesaving interventions, and individual assessment of triage category. The criteria within each of these categories were developed by a workgroup of experts representing national stakeholder organizations who used the best available science and, when necessary, consensus opinion. This article describes how the Model Uniform Core Criteria for Mass Casualty Triage were developed.