Wearable technology has matured from novelty step counters into serious health and fitness tools — but accuracy varies dramatically between products and use cases. This section validates wearable claims against reference equipment: heart rate sensors compared to chest strap monitors during interval training, GPS tracks measured against surveyed distance, SpO2 readings cross-checked with pulse oximeters, and sleep staging algorithms evaluated against user-reported sleep quality. We test battery life under realistic conditions — always-on display enabled, GPS tracking three times per week, notifications flowing throughout the day — rather than manufacturer-stated maximums. Beyond fitness metrics, we evaluate smartwatch functionality for daily productivity: notification management, voice assistant responsiveness, contactless payment reliability, and third-party app ecosystems. The guides address specific user profiles — marathon runners who need accurate pace and distance, gym-goers tracking strength progress, people managing chronic conditions with continuous monitoring — and recommend devices based on which sensors and algorithms actually deliver for each scenario. We also track the rapidly evolving landscape of non-invasive glucose monitoring, blood pressure estimation, and body composition analysis to separate genuine breakthroughs from marketing claims.