A review of the latest data on the problem of gall formation on plants under the influence of eryophyoid mites of the superfamily Eriophyoidea is given. Gall formation is induced in epidermal cells and includes a complicated growth reaction appearing as a response to the injection of mite saliva. The nature of gall-inducing agents is unknown. The role of such agents could be played by specific mite saliva factors affecting plant cell proliferation (e.g., plant hormones, probably secreted by mites) and microbial plant pathogens. Differences in the external gall structure are primarily determined by the mite species. Variability of galls is limited to several basic shapes that had probably appeared as convergent structures in phylogenetically remote plant hosts. A new renovated classification of types of damage caused by eryophyoid mites on plants is assumed on the basis of the following characters: the number of plant organs involved in the pathological process, duration of the existence of the damage and of the mite colony inside it, and localization of damage on the plant (topography). A step-by-step program for further studies of gall formation is substantiated.