The natural wood appearance of many species can be affected by a variety of undesirable "non-microbial" discolourations, which reduce the value of wood products. In contrast to sapstain, caused by fungi, prevention of these discolourations has rarely been demonstrated in practice. Discolourations of hem-fir have become an economically important problem with the move towards increased kiln-drying of the wood species mixture and added-value products in which discolourations are less tolerable. A literature review was done to survey both general information on "non-microbial" discolourations and more specifically information on discolourations of western hemlock and amabilis fir. Although discolouration of hem-fir lumber has been a puzzle for many years, knowledge of its cause(s) is rudimentary. Most research into hem-fir discolourations has been conducted on only a few wood samples. Although polymerization of wood extractives has been proposed as the probable cause, involvement by bacteria and fungi are also suggested in the literature. Other factors involved in discolourations of other wood species, such as factors inherent in the living tree, season of tree felling, post mortem changes and log age and storage, are reviewed. The nature of specific wood extractive chemicals and the significance of other contributory factors need to be understood before preventive treatments can be devised to maintain the natural colour in hem-fir products. Research recommendations include microscopic and histochemical techniques.