The field of photonic crystal fibers was pioneered in the late 1990s by Philip Russell, who developed this novel type of optical fiber with the ability to confine light in its core. With either a solid or hollow core, photonic crystal fibers have been revolutionizing optical communications ever since. This book presents the fundamentals of various guidance mechanisms and state-of-the-art applications in hollow core photonic crystal fiber (HC-PCF) technology. It discusses the unique characteristics of HC-PCF, which significantly reduce the loss experienced in conventional fibers, and shows how they are ideally suited for a broad range of applications, from lasers to fiber-optic sensors and power delivery. The authors examine the challenges of HC-PCF becoming the next generation of high powered fiber lasers with improved performance and reliability over today's optical fibers. They elucidate the principles that underlie its ability to confine gases and light together, discussing how this property is leading to a new paradigm in the study of gas-laser interactions. The book also reveals why HC-PCFs are such an excellent platform for exploring physics of photonic bandgaps.