This chapter argues that Photonic-Crystal Fibers' (PCF) diversity of features makes feasible a large number of new and improved applications in the sensing domain. The chapter presents PCF characteristics, concerning geometry and light guidance mechanisms, in order to understand it is based sensing systems. The appearance of PCF was a breakthrough in fiber-optic technology, particularly in the sensing domain. PCF characteristic structure enables a considerable enhancement in fiber design flexibility, providing a new breach for new and enhanced sensing solutions relative to the situation where the choice was limited to the standard Optical fiber (OF) technology. OFs are efficient solutions for sensing due to their high sensitivity, small size, robustness, flexibility, ability for remote monitoring, and multiplexing. Another property of great importance for sensing applications is its aptitude for operational work in the presence of unfavourable environmental conditions such as noise, strong electromagnetic fields and high voltages, nuclear radiation, explosive or chemically corrosive media, and high temperatures.