Modification of wood by altering its chemical structure is a way of improving its properties. In Europe research and development in this field has become increasingly important. A primary focus of research is to improve the biological durability of wood but also other properties are very important. These include hardness, hydrophobicity, strength, colour, and fire retardancy. With wood modification it is increasingly possible to tailor wood properties for a wide range of end products. Examples of wood modification are acetylated wood, making the wood UV resistant or lumen filling with a substance, usually a resin. These treatments might increase strength properties and slow down the process of water (vapour) uptake, but they do not change the sorption behaviour of the wood over a longer period of time. Bulking to fill the cavities in the cell wall as well as the cell lumen tend to reduce the swelling and shrinkage of the wood might even have a beneficial effect on the long-term sorption behaviour. Modifying treatments are usually the most effective. The chemical structures of cell wall components (lignin, cellulose and hemi-cellulose) are altered and covalent bonds are formed. Modification (in a stricter sense) can be divided into heat treatments, chemical modification and enzymatic treatments. Many properties can be improved permanently, in particular durability, dimensional stability and reduced equilibrium moisture content. The chemistry involved is not particularly complicated. However, the process technology for modification of solid wood is quite sophisticated. It is of paramount importance to combine the right chemicals with the optimum process conditions, at the correct place in the wood structure at the right time. In comparison, the modification of fibre materials is less complex.