Optical sensing systems for medical diagnostics have improved significantly in recent years. Many systems are now commercially available and provide greater reliability and sensitivity and/or improved responsiveness than previous systems. A key to the technology improvement has been an increased variety of optical fiber designs available for each optical system. The two primary types of fibers used in biomedical sensors are all-plastic and silica-core glass fibers. Each fiber family is identified and differentiated. The parameters discussed include sizes commercially available, numerical aperture ranges, spectral attenuation and ranges, mechanical strength and flexibility, and temperature compatibility. Data for six fiber types is presented. The applications of venous catheter oximeters and arterial blood gas analyzers are also discussed. Common design considerations and optical fiber limitations are reviewed. The tremendous diversity in optical fiber systems requires a large selection in optical fiber technologies. The design engineer must carefully evaluate the optical, environmental, and mechanical requirements of a biosensor system in order to choose the most appropriate optical fiber.