Abstract: The present invention relates to photonic crystal fibers, in particular to photonic crystal fibers that guide light by virtue of a photonic band gap. It is an object of the present invention to provide such fibers that are formed from materials having higher refractive indices than silica. The object is achieved by an elongate waveguide comprising a core with a relatively low refractive index and a photonic band gap structure that can substantially confine light to the core, the structure comprising elongate regions of relatively low refractive index interspersed with elongate regions of relatively high refractive index wherein the band gap resides above the fifth photonic band of the band gap structure. Alternatively the photonic band gap may have a V0-value in the range 130?V02?160. Alternatively the waveguide has a lattice pitch ? and the elongate regions have a transverse dimension d, wherein the photonic band gap has a normalized centre frequency k0? in the range 5?k0??6, when d/?=0.