This chapter introduces the basic principles of optical fiber sensing and provides a brief overview of some of the different types of fiber sensors developed all through the years. One of the biggest advantages of fiber sensors is that it has the capability to measure a wide range of parameters based on the end user requirement, once a proper fiber sensor type is used. Given the advancement in the development of optical fibers and instrumentation, the development and measurement capabilities of optical fiber sensors have also increased and are currently being employed in many structural health monitoring (SHM) and biomedical applications. As optical fiber sensors operate by modifying one or more properties of light passing through the fiber, they can be broadly classified as extrinsic or intrinsic. Wavelength-modulated fiber sensors exhibit a change in the propagating optical wavelength when interacted by an external perturbation. The chapter discusses descriptions of fiber sensors based on modulation methods, spatial positioning, and measurement parameters.