This chapter outlines the key principles and parameters which describe and define the operation of optical waveguides and fibres. The ways in which propagation through optical fibres affects the properties of the guided waves are discussed, including dispersion and non-linear effects. A further distinction arises depending on the index distribution within the guide. The simple planar guide, with a ray optics approach, provides a useful starting point for understanding the key properties of optical waveguides and fibres in general. The cut-off thickness for a mode is the minimum thickness which will support that mode. Waveguides which are made by diffusion or ion-exchange processes generally have a graded-index core. The matrix method is particularly useful for analysing multi-layer and/or graded index structures. A graded index structure is approximated by a large number of layers over which the refractive index is assumed constant.