Photonic Crystal Fibres (PCFs) have appeared as a new class of optical waveguides, which have attracted large scientific and commercial interest during the last years. PCFs are typically microstructured silica waveguides with a large number of air holes located in the cladding region of the fibre. The size and location of these air holes allows for a large degree of design freedom within optical waveguide design, and PCFs with properties tailored for fibre lasers, airguiding fibres, nonlinear fibres, hybrid fibres etc. have been demonstrated. Further, the existence of air holes in the PCF gives the possibility of propagating light through air, or alternatively allows access close to the fibre core for interactions with new materials placed in the air holes. This makes a well controlled interaction between light and material possible.