The aim of this book is to place at the disposal of the interested public (students of the natural sciences, biologists, agronomists, entomologists, etc.) an elementary but richly illustrated survey concisely describing the most common and biologically interesting galls. They are generally presented together with their causative agent, following the systematic classification of the parasites. In certain cases, the morphological presentation has been rounded out with biological details or even a brief anatomical and cecidogenetic study, manifesting the parasite's mode of action and the plant's coadaptive reaction. This book is not a catalogue, such as those of Houard (1908-1940) for the different continents, of Buhr (1964-1965) for central and northern Europe, or of Felt (1940) for North America. It is rather a systematic and biological account, more reminiscent of but having a more reduced and modern format than the not specifically cecidological work of Sorauer (Handbuch der Pflanzenkrankheiten, 1925-1931) and, from a cecidological perspective, the works of Küster (Die Gallen der Pflanzen, 1911) and Ross (Praktikum der Gallenkunde, 1932). Moreover, since the last treatise on cecidology, that of Mani (Ecology of Plant Galls, 1964), appeared over twenty years ago, a review incorporating more recent findings has become necessary. Although this book is more up to date, it cannot claim to be complete, all the more so because we have intentionally given it the form and content of a survey. For further knowledge, the reader will benefit by referring to the above-mentioned works. Concerning the anatomy and cecidogenesis, i.e., development, of galls, it would be advantageous to consult Küster (Anatomie der Gallen, 1930) and especially the quite recent treatise entitled Anatomie des galles by Meyer & Maresquelle (1983).