▪ Abstract  The development of microstructured optical fibers has led to the realization of many optical properties in fiber form that were not previously attainable. This chapter reviews the background to this work and overviews both the fundamentals of and progress in fabricating and modeling these structures. Until relatively recently, most of the work in this field was based on silica glass; this chapter provides an update on progress in developing microstructured fibers from other materials such as soft glasses. Some of the key applications of microstructured fibers, including nonlinear fiber–based devices and fibers for high power light delivery, are also reviewed.