This chapter introduces the practical application of diagnostic sensors within various systems. Section 9.1 delineates the taxonomy and operational principles of medical sensors, facilitating the detection and subsequent quantitative or qualitative analysis of pertinent data. In Sect. 9.2, emphasis is placed on in vivo diagnostics, encompassing a comprehensive overview of implantable cardiac monitors such as artificial pacemakers, and auditory prostheses like cochlear implants. This section delves into their historical evolution, operational mechanisms, and pertinent application scenarios, while also elucidating ongoing developmental trajectories aimed at enhancing biocompatibility and minimizing energy consumption. Section 9.3 pivots toward in vitro diagnostics, spotlighting cutting-edge immunodiagnostic techniques like ELISA and immunofluorescence analyses, alongside molecular diagnostic methodologies such as PCR and gene sequencing technologies. These methodologies facilitate the detection of specific biomarkers and afford precise observations via microscopy and allied instruments, thereby augmenting the accuracy of diagnostic outcomes. Section 9.4 introduces the realm of minimally invasive diagnostics, characterized by its benefits of diminished risk, expedited recovery, and reduced trauma. This segment predominantly expounds upon the operational principles and practical applications of microneedle diagnostics, basic low-frequency electrical stimulation, and ultrasonic diagnostics. These modalities afford insights into specific anatomical functions, facilitate status assessment, and prognosticate recovery trajectories. In Sect. 9.5, the discourse shifts toward wearable physiological index monitoring and diagnostics. Encompassing wearable technologies such as patches and wristbands, this section delineates the monitoring and diagnostic capabilities pertaining to physiological parameters like electrocardiogram (ECG), electromyogram (EMG), and blood oxygen saturation. Furthermore, it elaborates on wearable biochemical parameter monitoring and diagnosis, encompassing analyses of sweat, interstitial fluid, and urine. Wearable diagnostic technologies, distinguished by their swift detection capabilities and cost-effectiveness, predominantly serve in the early screening of latent health risks. Section 9.6 elucidates the paradigm of smart integrated diagnostics, offering an exhaustive examination of multimodal diagnostics and the utilization of smart sensors within the healthcare domain. Additionally, it posits prospective developmental trajectories such as miniaturization, energy efficiency, integration with artificial intelligence, and multimodal perception.