By offering greatly enhanced control of light compared to conventional step-index structures, photonic crystal fibres are radically improving the performance of linear and nonlinear fibre devices, including gas-Raman cells, super-continuum generators, soliton systems and cladding-pumped lasers. By offering greatly enhanced control of light compared to conventional step-index structures, photonic crystal fibers are radically improving the performance of linear and nonlinear fiber devices, including gas-Raman cells, super-continuum generators, soliton systems and cladding-pumped lasers.