The anils of ortho-hydroxybenzaldehyde comprise a chemical system which exhibits the properties of solid state photochromism and thermochromism. These properties are exclusive in this system and therefore there is a problem which may be described as the relation between photochromism and thermochromism on the one hand and crystal structure on the other. Thus in this work the topochemical factors responsible for these properties are explored and "structural engineering" approach are applied in order to prepare solid anils which are photochromic and/or thermochromic at will. Literature data on crystal structure and photo-behaviour of anils are presented and discussed under the same light in order to have a more complete understanding of the problem.