© 2018 Scrivener Publishing LLC. This chapter highlights top-down and bottom-up approaches to generate ultra-fine cellulose fibers of nano-scale dimensions, micro-porous, meso-porous and sheathcore hybrid structures as well as surface functionalized fibrous materials. Versatile solvent systems have been established to efficiently dissolve cellulose acetate to enable robust electrospinning into homogeneous 1D nanofibers and submicron size fibers that could be easily converted from amorphous to moderately crystalline cellulose II via alkaline hydrolysis. By pairing with either compatible or incompatible polymers, hybrid and nanocomposite fibers as well as porous fibers may be engineered. Surface reactions, grafting and electrostatic deposition can further offer surface functionalization for controlled hydrophobicity, stimuli-responsive behaviors and bound enzyme catalyses. Highly crystalline IΒ nanocelluloses with varied geometries and surface chemistries have been efficiently derived via chemical means and/or shear forces in nanorod and nanofibril forms that have been shown to exhibit unique dispersing and emulsifying properties for oil-water emulsions, coagulants for microbes as well as templates for nanoparticles and nanoprisms. These nanocelluloses can be facilely assembled into new fibrous structures, super-absorbent hydrogels, films and amphiphilic to hydrophobic aerogels for applications, such as oil recovery and separation, water purification, etc. While ultra-fine fibers from these two approaches share some common fibrous morphologies, their crystalline structures, thermal behaviors, surface chemistires, reactivity and chemical functionalities are distinctively different, offering a wide range of strategies for fabricating cellulose nanofibers with tunable functional characteristics for novel materials and advanced composites.