Machine strength grading of timber has been used commercially for at least 30 years, but, for many people involved with the process, it has remained a specialised subject. Timber enters a grading machine at one end and leaves the other having been given a strength class with no obvious means of how this is achieved. Without knowing the theory and practice of grading, misunderstandings are likely to arise. The main aim of this Digest, then, is to improve knowledge of the process of machine strength grading and to highlight the benefits. It also explains some aspects of statistics, the concept of distribution, and modification and adjustment factors which are integral to timber grading practice. 12 pages.