This book is based on Herbert Buhr's seminal keys of 1964-1965 (Bestimmungstabellen der Gallen (Zoo- und Phytocecidien) an Pflanzen Mittel- und Nordeuropas) and augmented with Houard's 1908-1913 work on Southern Europe (Les Zoocécidies des Plantes d'Europe et du Bassin de la Méditerranée). The authors have updated this with the research of a new generation of cecidologists, significantly expanding our knowledge of plant galls and their distribution. The 9,000 galls and malformations described by Buhr and Houard have been updated and 1,250 new galls described in more recent literature have been added. The nomenclature of both gall inducers and host plants has been updated, information about specific groups of gall inducers has been reviewed, and new insights have been added by a team of specialists. Moreover, they collected distribution data for the whole of Europe and, where available, adjacent areas.