Timber modification is an emerging commercial reality. Timber that has been acetylated, thermally modified or furfurylated is now readily available. However, the assessment of the effectiveness of each modification type in enhancement of durability, and therefore the effect on service life, has yet to be fully established. The paper includes a summary of the key points necessary to understand the mechanisms of protection given to wood by such modification. This paper also brings together the current work on the assessment of the durability of three types of modified wood and the effects that each has on the service life of timber structures, and addresses the harmonisation of standards in this field. This will allow for future development of appropriate standards for utilising these novel timber products, and therefore improve service life prediction and confidence when designing with these materials.