The introduction of InGaN materials have made LEDs the light source of choice for diagnostic and photo-therapy applications from the UV to the near infrared. This course provides an overview of LED structures, packages, and performance and their application in non-invasive diagnostics and photo-therapy. It compares the spectral brightness and tissue coupling efficiency of LEDs, lamps, and lasers for large area illumination (photo-therapy), fiber-coupled spot probes and imaging systems. Design of pulse oximetry probes to minimize wavelength variation errors and tissue physiological variations is explained. LED array configurations, and modulation-demodulation detection schemes for imaging optical biopsy systems are explored.