Since the development of the first modules, hollow fibers have totally revolutionized the world of membranes—thanks to the new technological breakthroughs and innovative materials discovered. The importance of a book putting hollow fibers in the spotlight owes to this type of configuration being more and more appreciated, particularly in large-scale applications. Moreover, the advent of nanofibers has injected new vitality in biomedical research, air and water separation and filtration processes, and emerging areas of nanotechnology. This book singles out and highlights the unique properties that hollow fibers and nanofibers display in the fields where they already represent the dominant configuration and where their full exploitation is still hindered by economic and technological constraints.