This book provides an overview of several topics concerning the design, fabrication, and application of optical fibers, namely in the areas of communication systems, sensing, and photonic devices development. It consists of ten chapters. The first two chapters are concerned with different kinds of problems that can affect the performance of advanced optical fiber communication systems. Chapter One describes the polarisation-mode dispersion (PMD) phenomenon and discusses PMD-induced pulse broadening, as well as different compensation techniques, including the case of soliton transmission systems. Chapter Two provides a review of the main limitations imposed by nonlinear effects on the performance of both single-channel and multi-channel optical fiber communication systems. Due to continued internet growth, the worldwide traffic demand for long-haul networks has nearly exhausted the capacity limits of conventional single-mode fiber. Space division multiplexing (SDM) technologies have become a promising approach to resolve this bandwidth crunch. Chapter Three presents an overview of the state-of-the-art SDM-based communications systems, considering both few-mode fibers (FMFs) and multicore fibers (MCFs). Chapter Four discusses several FMF-based nonlinear processes in the context of different optical communications and sensing applications. Optical fibers have been used during the last decades to realise various types of photonic devices. Chapter Five presents a study of the performance of several fiber-based devices used in the areas of optical communications and sensing. Chapter Six provides a review of the cavity ring-down technique, which looks like a very promising technique and has been vastly employed in several areas of research. Microstructured optical fibers (MOFs), also called photonic crystal fibers (PCFs), represent a new class of optical fibers that are characterised by the fact that fiber cladding presents an array of embedded air holes. They can offer different possibilities for the fiber optic sensing field, namely for the fabrication of fiber in-line modal interferometers (MIs). Chapter Seven describes the fabrication, operating principles and sensing applications of MOF-MIs. Chapter Eight discusses several phenomena concerning the ultrafast dynamics of femtosecond pulse propagation in gas-filled kagomé hollow-core PCFs, namely pulse compression, supercontinuum and UV light generation. Chapter Nine analyses the fundamentals of twisted clad guides, considering various forms of microstructured mediums. Finally, Chapter Ten provides a detailed review of the most recent developments in the field of nano-structured glass-based optical fibers fabrication. The application of such kinds of erbium and thulium doped phase-separated dielectric nano-particles-based fibers, as well as silicon nano-particles doped fibers towards the development of fiber lasers, optical amplifiers and broad band light sources is envisaged.