Vital signs are the cornerstone of patient assessment and provide valuable insight into a patient's physiologic status. While heart rate, blood pressure, respiratory rate, oxygen saturation, temperature, and end-tidal carbon dioxide are routinely obtained, understanding what these measurements represent is essential for accurate clinical decision-making. This session explores the theory of vital signs by examining the physiologic principles behind each measurement and their role in identifying health and disease. Participants will review how normal vital sign values are established through population-based research and clinical evidence, while recognizing that "normal" varies based on factors such as age, baseline health, medications, and environmental influences. The presentation will emphasize moving beyond memorizing reference ranges to interpreting vital signs within the context of the patient's overall condition. Using a pathophysiology-based approach, attendees will learn how changes in vital signs reflect the body's compensatory mechanisms and underlying disease processes. Through practical case examples, participants will connect abnormal vital sign patterns with alterations in perfusion, oxygenation, metabolism, and homeostasis, allowing them to better recognize early patient deterioration and prioritize treatment decisions. Whether evaluating medical or traumatic emergencies, this presentation provides EMS clinicians with a practical framework for interpreting vital signs as dynamic indicators of physiology rather than isolated numbers. By understanding the "why" behind the values, providers will enhance patient assessment, strengthen clinical reasoning, and improve decision-making in the prehospital environment.