The topic of this thesis is the enzymatic modification of cellulose by the laccase/TEMPO system. 2,2,6,6-Tetramethylpiperidine-1-oxyl (TEMPO), which has so far been applied mainly in chemical oxidation systems together with hypohalite to effect oxidation of low-molecular weight carbohydrates and cellulosics, is also an effective mediator in combination with laccase. The effect of the laccase/TEMPO system on cellulosics with regard to molecular weight changes and introduction of functional groups has been studied using gel permeation chromatography in combination with selective fluorescence labeling of oxidized groups (carbonyls and carboxyls). LMS treatment caused uniform oxidation of the cellulosic material, also in the high molecular-weight area, which is a non-typical behavior compared to conventional chemical oxidations of cellulose. The chemoenzymatic approach was compared to the well-known chemical approach employing TEMPO/hypohalite. The performance of three different laccase preparations was studied with regard to applicability in organic solvents for synthetic-organic carbohydrate and cellulose chemistry: soluble (lyophilized) laccase, immobilized enzyme and crosslinked enzyme crystals (CLECs). The latter showed a significantly higher activity compared to the immobilized and the lyophilized variants. We also examined the role of 4-acetamido-TEMPO as the mediator part of LMS in cellulose oxidation systems, with a special focus on possible side reactions, degradation pathways and the chemical structure and fate of the TEMPO-derived species. Degradation was strongly dependent on the pH value of the reaction mixture. Under acidic conditions (pH 5), 4-oxo-TEMPO is formed by an oxidative deamination process. In alkaline media, a similar process proceeds, but is immediately followed by a Favorskij rearrangement to form a pyrrolidine-3-carboxylic acid (PROXYL) derivative. Eventually, a novel LMS system for the regeneration of NAD(P)+ from NAD(P)H for use in dehydrogenase-catalyzed reactions was studied.