This chapter presents the major internationally validated and widely implemented emergency department triage systems and describes their core structures and practical implications. Despite having emerged from different health-care contexts, these models share a common operational architecture: a five-level urgency classification, explicit criteria to support priority assignment, time-to-assessment targets, and minimum requirements for training, audit, and quality monitoring to preserve reliability over time. The chapter highlights the elements that make these systems interoperable in practice: the use of clinical observation and vital parameters as discriminators, linkage of triage levels to defined time goals, and the need for reassessment while waiting, thereby enabling benchmarking, clinical governance, and continuous improvement. It then introduces the internal logic of each model through its defining design choices.