Risk stratification is presented as the methodological backbone of contemporary emergency medicine and the core logic underpinning safe triage decisions. This chapter defines risk stratification as the assignment of a graded probability of adverse outcomes within a defined time horizon and explains why, in the Emergency Department (ED), urgency must be understood not only as current severity but also as the likelihood of deterioration, need for critical interventions, admission, readmission, or death. By framing ED care as early, data-poor, time-compressed decision-making, the chapter clarifies how structured risk thinking reduces prognostic error, supports proportional allocation of scarce resources, and increases clinical safety. It then examines the foundational relationship between stratification and triage: triage operationalizes risk into time targets and pathways, while stratification provides the prognostic rationale that justifies prioritization and reassessment. Finally, the chapter discusses practical and educational implications.