Fiber-optic technology emerged originally for applications in data transmission and telecommunications. However, sensors based on fiber-optics have been developed rapidly because of their excellent sensing performances and capability to function in remote and harsh environments. The usage of fiber-optic sensors has flourished in many fields over the past 30 years due to the fiber-optic's inherent advantages: cost-effectiveness, miniaturized size, light weight, and immunity to electromagnetic interference. This work reviews the fiber-optic sensors based on Bragg gratings, long period gratings, interferometers, surface plasmon resonance, fluorescence, and light diffusion. Brief theory of sensing principle, fabrication method, applications, advantages and disadvantages of the different fiber-optic sensors, are addressed. Recent progress in numerous sensing fields, including environmental, industrial, and biomedical are discussed for each class of fiber-optic sensors. The review highlights the methods and techniques used to overcome the sensing challenges. Finally, prospect of future developments of fiber-optic sensors is summarized.