Light-induced fibres: light guiding light When the refractive index changes with the intensity of light, a number of interesting nonlinear phenomena can occur. In this chapter, we concentrate on "powerful" nonlinear situations, when light guides itself in a homogenous medium without any intervening core-cladding structure. In other words, light itself induces its own guiding structure. This fascinating phenomena has been studied theoretically with many confirming experiments. Light guiding light offers a multitude of devices wherein light becomes the master of its own destiny and allows for the possibility of rewritable circuitry. There are various approaches for describing highly nonlinear phenomena like self-induced waveguides. One is formal and lacks intuition. This approach is to solve Maxwell's equations from the outset with an intensity-dependent refractive index, completely ignoring the physics of the earlier chapters. The other approach is to build on our earlier chapters, by recognising that a light indued waveguide is a waveguide just like those we have already studied; the only difference being that its refractive index (and hence its V value) is now dependent on the intensity of light.