Optical fiber sensors based on the amplitude modulation technique are discussed. According to their basic working principle, these sensors are classified into three classes: spectrometric, transducer, and all-fiber sensors. Examples of fiber-optic sensors developed for biomedical and environmental applications are given. Typical parameters of biomedical interest, which are conveniently monitored by fiber-sensors, are: temperature, pressure, blood flow, pH, pO2, pCO2, glucose, and immunological parameters. Other parameters of interest for environmental monitoring are CH4, other hydrocarbons polluting air and water, and trichloroethylene and chloroform.