Plant cell walls consist of a wide range of polysaccharides, proteins, and phenolic compounds. They surround all cells and constitute an exoskeleton that governs the size and shape of the plant. How plant cell walls are constructed to confer high strength while remaining dynamic is largely unknown. The fundamental building blocks of cell walls are cellulose microfibrils (3-4 nm wide and several micrometers long). These building blocks can be derived from cell walls by chemical and mechanical methods. The resulting elements are referred to as cellulose nanofibers (CNF). The large surface area combined with properties of cellulose such as reactive hydroxyl groups and high strength-to-weight ratio (8 times higher than steel) makes CNF a potential building block for designing novel materials. Nevertheless, a major bottle neck for realizing many of the industrial applications of CNF is its high production-costs due to high water consumption, use of harsh chemistry, and high energy-input. The first part of this thesis describes novel methods for deriv ing CNF from agro-industrial waste, such as sugar beet and potato pulps. The abundance of these pulps, originating from industrial production of starch and sugar, constitutes a sustainable source of CNF. Due to highly hydrated cell walls within these raw materials (compared to wood), we have explored the use of enzymatic treatments, instead of chemistry, to remove the non-cellulosic fraction. The methods developed herein show great potential for producing high quality CNF with reduced water consumption and less toxic effluent, thus making the material more cost-effective and environmentally friendly. In the second part of this thesis we have additionally explored the use of CNF as model substrates to probe expansin activity and cellulose-degrading enzymes. It appears that CNF-materials have a utility in this respect due to its similarity to cellulose in muro and resemblance to plant cell walls.