Pseudostructures, which are not real properties of electron microscope specimens but are introduced by the action of the electron beam, are described. Examples are given from samples of tetracopper calcium oxychloride, precipitates from slowly hydrolized ferric chloride solutions, and tungsten oxide to illustrate some of the effects which may occur. The polymorphism of these substances, particularly of tungsten oxide, is discussed. The major effects which contribute to the pseudostructure formation are melting, evaporation, change of crystalline states, increase in degree of crystallinity, and migration of material. Reasons are given for concluding that the primary cause of specimen changes is to be found in the heating effect of the beam. The possibilities of secondary causes are also considered. Artifacts may also be introduced into bacterial specimens. Observed changes in such samples are described briefly and examples are given.