This is the first of an intended series of papers on the family Thelypteridaceae, especially in the Old World, based on studies made in preparation for an account of the family for Flora Malesiana. At a later stage I will give a formal statement of the characters of the family, but here I must point out that a former statement of mine (Holttum, 1947, p. 130) needs to be modified as regards characters of scales and hairs. Scales. These may bear either marginal or superficial unicellular hairs, or both, or perhaps in rare cases none; the hairs may be acicular, or capitate, or spherical and glandular.