Photonic crystal materials and waveguides have since their appearance in 1987 attracted very much attention from the scientific community. From being a more academia discipline, new components and functionalities have emerged, and photonic crystals have today started to enter the field of commercial devices. Especially the photonic crystal fiber (PCF) with its lattice of air holes running along the length of the fiber has matured, and the technology provides a large variety of novel optical properties and improvements compared to standard optical fibers. With respect to optical sensors, the photonic crystal structures have several important properties. First of all the wavelength-scale periodically-arranged material structures provide completely new means of fabricating tailored optical properties either using modified total internal reflection or the photonic bandgap effect. Secondly, the new materials with numerous micro- or even nano-scale structures and voids allow for superior mode control, use of polarization properties, and even more a the potential of close interaction between optical field and the material under test. The present paper will be using the example of the relatively mature photonic crystal fiber to discuss the fundamental optical properties of the photonic crystals, and recent examples of their use as optical sensors will be reviewed.© (2004) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.