Growing global populations and the rapid increase in older adults are driving healthcare costs upward. In response, the healthcare system is shifting toward models that allow for continuous monitoring of individuals without requiring hospital ad-mission. Advances in sensing technologies, embedded systems, wireless communica-tion, nanotechnology, and device miniaturization have made it possible to develop smart systems that continuously track human activity. Wearable sensors can monitor physiological indicators and other symptoms, helping to detect unusual or unexpected events. This enables timely assistance when it is needed most. This paper outlines these challenges and reviews recent developments in wearable sensor–based human activity monitoring systems. The focus is on health monitoring applications, including relevant biomarkers, wearable and implantable sensors, estab-lished sensor technologies currently used in healthcare, and the future prospects and challenges involved in researching, developing, and applying these sensors to support widespread use in human health monitoring.