Material degradation and a decrease of fuel quality are common phenomena when storing biomass. A magnitude of 7.8% has been reported to degrade over five months when storing spruce wood chips in the winter in Central Europe. This thesis presents a theoretical study of biomass storage. It includes investigations of bio-chemical, chemical and physical processes that occur during storage of chipped woody biomass. These processes lead to degradation caused by micro-activity, chemical oxidation reactions and physical transformation of water. Micro-activity was modeled with Monod kinetics which are Michaelis-Menten type of expressions. The rate expressions were complemented with dependency functions describing the impact of oxygen, moisture and temperature. The woody biomass was divided into three fractions. These fractions represent how hard different components of the wood are to degrade by microorganisms. Chemical oxidation was modeled as a first order rate expression with respect to the active components of the wood. Two different cases have been simulated during the project. Firstly, an isolated system with an initial oxygen concentration of air was considered. This case displayed a temperature increase of approximately 2˚C and a material degradation less than 1%. The second case considered an isolated system with an endless depot of oxygen. This case resulted in degradation losses around 0.45-0.95% in the temperature range between 65-80˚C during approximately 300 days of storage. The temperature increased slowly due to chemical oxidation.