In modern emergency care, triage systems are not merely operational tools to organize a waiting room; they represent the first clinical decision point and the threshold that shapes the entire diagnostic–therapeutic pathway. This chapter provides a systematic comparative analysis of the five major internationally adopted five-level triage systems, ATS, CTAS, ESI, MTS, and SATS, focusing on their conceptual foundations and structural differences rather than on superficial similarities. Although these models share an apparent common architecture, they originated within distinct health-care contexts and organizational priorities and, therefore, embody different interpretations of what “urgency” means in practice. The discussion emphasizes why professionals trained within a single national system may inadvertently interpret local methodological choices as universal principles, and why understanding cross-system differences is essential for safe implementation, meaningful benchmarking, and informed critical appraisal of the international literature.