Reviews current theories regarding fiber properties and their relationships to paper structures. Increased use of various microscopictechniques has greatly enlarged understanding of fiber structure. Interaction between the elements of fibers and water is shown to be the basic factor in paper formation, with the relationship between the free and absorbed water in the system ranking high in importance. Physical characteristics of individual fibers are believed to determine shrinkage and internal strain of the sheet. The consistency of fibers in water suspensions is thought to be an important variable in deter mining network strength and fiber flexibility. Modern theories indicate that, during the thickening and drying process, interfiber bonding occurs at a lower solids content than was formerly thought. The relative number of fibers free to dry individually determines the shrinkage in the sheet. Some work in the use of geometric models to -- AATA