Thermally modified wood (TMW) is currently produced commercially by a range of processes across many countries. A prerequisite of the commercial success is an efficient quality control (QC), and methods with this regard are discussed in this review. When direct measurement of the key attribute of the material is not feasible, QC is based on a suitably chosen physical or chemical "marker". A critical evaluation of currently applied markers reveals that most of them only provide data for comparative purposes for a particular species and/or over a narrow process range. Such markers do not allow making an objective judgment of quality, which is independent of process information or reference samples provided by the manufacturer. On the other hand, they can be very useful for monitoring product variability in the TMW factory and wood during the heat treatment. Recommendations for future development are the general validation of (combinations of) known TMW markers for different wood species and processes, resulting in (1) a reliable and fast laboratory QC method for given samples of unknown origin, (2) a simple and fast indicative QC test for end users, and (3) in-line product markers for feedback-controlled production.

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