With the development of nanotechnology, there has been great progress in the preparation of various nanomaterials. Among them, nanofibers have unique functionalities based on their nanoscaled diameter, large surface-to-volume ratios, and high molecular orientation or confined polymer chains inside the fibers. Nanofibers and their assemblies (e.g., membranes, nonwovens, and yarns) have recently attracted much attention in applications such as high-performance air and liquid filter media, strong and tough fibers, battery separators, electrode materials, protective clothes, composites, drug delivery systems, and biomaterial scaffolds for tissue engineering. This review highlights production methods, fundamental aspects of functional design of nanofiber materials (by controlling their fiber diameter, surface chemistry and topology, and internal structure of the nanofibers), and recent advances of nanofiber materials in their environmental and energy applications (e.g., air filter, separation membrane, and electrolyte).