Book excerpt: This book aims to provide expert guidance to researchers experienced in classical technology, as well as to those new to the field. A variety of perspectives on Photonic Crystal Fibres (PCFs) is presented together with a thorough treatment of the theoretical, physical and mathematical foundations of the optics of PCFs. The range of expertise of the authors is reflected in the depth of coverage, which will benefit those approaching the subject for a variety of reasons and from diverse backgrounds. The study of PCFs enables us to understand how best to optimize their applications in communication or sensing, as devices confining light via new mechanisms (such as photonic bandgap effects). It also assists us in understanding them as physically important structures which require a sophisticated mathematical analysis when considering questions related to the definition of effective refractive index, and the link between large finite systems and infinite periodic systems. This book offers access to essential information on foundation concepts of a dynamic and evolving subject. It is ideal for those who wish to explore further an emerging and important branch of optics and photonics.