Photonic crystal fibers, also known as microstructured or holey fibers, have recently generated great interest in the scientific community thanks to the new ways provided to control and guide light, not obtainable with conventional optical fibers. Proposed for the first time in the 90s, photonic crystal fibers have driven an exciting and irrepressible research activity all over the World, starting in the telecommunication field and then touching metrology, spectroscopy, microscopy, astronomy, micromachining, biology and sensing. This book is intended to provide an expert guidance through the properties of photonic crystal fibers, with a specific focus on the telecommunication aspects. The authors believe that photonic crystal fibers can revolutionize the field of guided optics and its applications, especially when considering signal processing and specific functions rather than the usage in long distance transmission. They provide a deep analysis of how the physical and geometrical characteristics of these new fibers can be tailored to achieve the goal of ad hoc performances, using the powerful numerical approach of the finite element method, and keeping in mind the possibilities and limits of photonic crystal fiber fabrication technology.