An introduction to the field of the photonic crystal fibre (PCF) is given and the basics are illustrated by the similarities with standard fibres. The theoretical introduction is followed by examples of the technology. Dispersion in the second order mode of a PCF is modelled with the aim to find the highest possible negative dispersion, useful for dispersion compensation. Dispersion of a PCF design for supercontinuum generation (SCG) is modelled. Dispersion engineering of the structure of the PCF shows how to turn power loss into extra signal gain, thereby significantly raising the efficiency of the involved nonlinear processes. A hollow-core photonic crystal fibre (HC-PCF) is used as a sensor for gas. It is filled with two gasses, 12C2H2 acetylene, and H13CN hydrogen cyanide, and the transmission spectra are subject for a discussion. A model for infusion speed of fluids to a capillary presented. It is verified in three different experiments. Two different schemes for the selective filling of a fibre core alone are devised, and launch of light - through the cladding, from the side of the fibre - is shown. Short pieces of hollow-core PCF guide light in their cladding, despite their band gap. Such cladding mode guidance is prohibited by coating the end facet of such fibres with half a micron of gold, and adverse cladding-guidance is reduced as much as 30 dB.