Factors affecting the swelling of cellulosic fibers are considered in this review. Emphasis is placed on aqueous systems and papermaking fibers, but the review also considers cellulose solvent systems, nanocellulose research, and the behavior of cellulosic hydrogels. The topic of swelling of cellulosic fibers ranges from effects of humid air, continuing through water immersion, and extends to hydrogels and the dissolution of cellulose, as well as some of its derivatives. The degree of swelling of cellulose fibers can be understood as involving a balance between forces of expansion (especially osmotic pressure) vs. various restraining forces, some of which involve the detailed structure of layers within the fibril structure of the fibers. The review also considers hornification and its effects related to swelling. The expansive forces are highly dependent on ionizable groups, pH, and the ionic strength of solution. The restraining forces depend on the nature of lignin, cellulose, and their detailed structural arrangements.