The transition from episodic clinical assessment to continuous physiological monitoring represents a transformative shift in biomedical engineering. This review comprehensively examines recent advancements in human-interfaced sensing systems, categorized into skin-attachable and implantable platforms. We analyze innovations in interface architectures designed to resolve the mechanical mismatch between rigid electronics and soft biological tissues. Key strategies include the utilization of elastomeric substrates, functional hydrogels, and high-performance nanomaterials to ensure mechanical compliance, biocompatibility, and stable electrical contact. Furthermore, this paper details diverse signal measurement modalities, ranging from electrochemical biosensors for metabolic biomarkers (e.g., glucose, lactate) to electrophysiological recordings (ECG, EEG, neural probes) and physical sensing (strain, pressure). Representative applications including smart contact lenses and wireless mouthguards highlight the potential of these technologies in real-world diagnostics. Despite significant progress in wireless telemetry and miniaturization, challenges regarding long-term biostability, enzymatic degradation, and the foreign body response persist. We conclude by discussing the future trajectory toward autonomous, closed-loop theranostic systems integrated with artificial intelligence and biodegradable materials, paving the way for ubiquitous, personalized health management.