A photodiode is a solid-state sensor that responds to light by producing a measurable electronic current. Operating as a quantum threshold detector, a photodiode will only detect photons that exceed a certain energy level (the bandgap energy of the photodiode material) while not detecting photons below this level. This article covers the basics of pn-junction, PIN, and avalanche photodiode operation, as well as the use of photodiodes in several example applications in biology and medicine. Photodiodes are attractive for biomedical applications because they are inexpensive compared with many other detection mechanisms, easy to use, small scale, readily available, and capable of operation in an array for parallel sensing. They are ideal for applications involving bed-side monitoring such as pulse oximetry, rapid diagnosis including point-of-care instrumentation based on lab-on-a-chip technology, and other instances where equipment benefits from greater miniaturization or portability. Semiconductor photodiodes are fabricated using similar methods used to produce transistors and integrated circuits and continue to evolve together with integrated circuits to be efficient and inexpensive photodetectors while maintaining their basic simplicity.