The theories of Metzger, who described the tree stem as a beam of uniform resistance, and of Gray, who claimed that the tree stem was not ‘fixed’ but anchored in a relatively weak substratum, are described and discussed. The equation describing Gray's taper line is D2 = a—bH , where D is diameter at height H and a and b are constants. This equation was found to give satisfactory results when tested on seventy-three trees from the coastal forest region of British Columbia and twenty-two plantation-grown Scots pine trees from the south of England. Taper, as expressed by the regression coefficient, b , is not affected by stand age or site index and varies very little between the species tested. Over ninety-five per cent. of the variation in b could be removed by a combined variable diameter breast height (d.b.h.) 2 /(total height). Taper increases throughout the life of the tree as long as it remains in the dominant crown class but begins to decrease with age as the trees become dominated. The variation of taper with thinning régime was studied on the Scots pine data. It was found that taper increased most rapidly with age in the heaviest thinning grade and least rapidly in the lightest grade.