Research and development are actively underway on nanocellulose, a crystalline assembly obtained by top-down chemical or physical processing of natural cellulose-containing raw materials. This includes materials like cellulose nanofibers and cellulose nanocrystals. In parallel, cello-oligosaccharides are gaining attention as novel functional materials. These cellulose oligomers have a significantly lower molecular weight than naturally derived cellulose, typically around several thousand (a degree of polymerization of 7–20). Despite their low molecular weight, they are water-insoluble and capable of crystallization through self-assembly. These cello-oligosaccharides can be synthesized easily and with high purity through enzyme-catalyzed polymerization using unprotected saccharides as raw materials (Fig. 144.1) [1–5]. A key advantage is the use of aqueous solvents during synthesis, which leads to the in situ crystallization of cello-oligosaccharides once they reach a specific degree of polymerization. This process constructs unique crystalline assemblies on the spot. Enzymes such as cellulase, cellodextrin phosphorylase, and cellulose synthetase have been employed for this synthesis. Notably, using cellodextrin phosphorylase allows for the introduction of various functional groups at the reducing end of the cello-oligosaccharides, opening up attractive possibilities for functional materials [2, 3, 5]. While naturally derived nanocellulose typically exhibits fibrous or rod-like forms, cello-oligosaccharide assemblies can take on various nanoscale morphologies that are difficult to achieve with natural sources. These include nanosheets, nanoribbons, and helical structures. Due to their artificial, bottom-up construction, these crystalline cello-oligosaccharide assemblies are also referred to as synthetic nanocellulose, contrasting them with their naturally derived counterparts. Although research into their use as functional materials began only about a decade ago, synthetic nanocellulose has already demonstrated a wide range of potential applications across various fields.