Introduction In optical sensing, the most recent and important enabling technologies are based on either the longitudinal or transverse structuring of fibers. Bragg gratings are a typical example of the longitudinal structuring of fibers, while microstructured fibers [e.g., photonic crystal fibers (PCFs)] are the promising enabling technology in terms of transverse structuring of fibers. Microstructured fibers provide materials with dispersion characteristics unattainable with conventional materials, as well as otherwise unfeasible physical characteristics that can be tailored to specific sensing applications. Functional materials can be realized on the side- or end-face of fiber with many techniques, including sputtering, thermal evaporation, spin-coating, and so on. The combination of fiber optics with nanostructure technologies and functional materials offers great potential for the realization of novel sensor concepts. Miniature optical fiber sensors with functional coatings and thin films as sensitive elements could open new fields for optical fiber sensing applications. Functional coatings work as sensitive elements and transducers to get response and feedback from the environment, while optical fibers are employed here as carriers of sensing signals.