Summary form only given. As optical fiber performance has been refined over the past three decades, so the limits of what "conventional" fibers can do have become widely accepted. Over the past few years, researchers at several institutions have looked again at a fabrication technology first investigated in the 1960s, that of "single-material fibers", and re-interpreted the possibilities of that technology in the light of present trends in optics. Photonic crystal fibers use a single two-dimensionally microstructured material to form a fiber waveguide. By considering these materials as photonic crystals, we can develop new ways of thinking about light confinement in optical fibers. This has led to progress in two directions. Firstly, it has been recognized that true light confinement (and hence waveguiding) can be achieved without total internal reflection (TIR) if one can structure the fiber cladding to form a photonic bandgap material. This means that it is possible to form previously unthinkable fiber waveguides, as in a fiber in which light is truly guided single-mode in a small air core. Secondly, even when using TIR, the possibility of 2D microstructuring of the fiber cladding coupled with the high index contrast of single-material fibers means that some of the limitations of conventional fiber waveguiding no longer apply.