In the past decade a broad range of sensing applications have been reported utilizing optical-fiber-based sensors. These applications range from the measurement of a variety of physical parameters to the quantitative determination of chemical and biochemical species. The acceptance of many of these sensors by the process monitoring and control industry has not met manufacturer's expectations. This paper describes the special requirements and concerns which must be addressed in implementing fiber-optic sensors in process environments. Topics include sensor specificity and interferences, environmental constraints, long term stability, sensor calibration methods, sensing mechanisms, sensor system design for self-calibration, and performance characterization in process environments. Special requirements for single point and distributed fiber-optic sensing applications including temperature, pressure, strain, humidity, and chemical concentration are described. Methods are discussed for designing robust fiber-optic sensors which remain calibrated for extended periods of time. Also included is a discussion of self calibration methods for use in reflective mode intensity, analyte and distributed based fiber-optic sensors.