In-hospital triage emerged from military medicine and progressively evolved into a core clinical–organizational function of modern Emergency Departments (EDs). This chapter outlines the historical transition from battlefield prioritization to hospital-based triage, driven by rising ED demand, crowding, and the need to protect patients from the harmful consequences of delayed care. Modern triage is defined by rapid, essential assessment under severe time constraints, reliance on focused history, clinical observation and vital signs, and the assignment of reproducible urgency levels linked to target times for medical evaluation. Beyond operations, triage embodies distributive justice in conditions of relative resource scarcity, ensuring transparent and clinically grounded prioritization rather than “first-come, first-served” access. The chapter also frames triage within ED systems theory through the input–throughput–output model, highlighting how congestion and boarding threaten safety and how triage acts as an early filter to mitigate variability and optimize internal processes. Finally, it describes the methodological shift since the 1990s toward evidence-based, standardized five-level systems and clarifies the crucial distinction between triage (temporal prioritization) and streaming (routing to the most appropriate care pathway).