Until recently, the system of classification of the Psylloidea was discussed exclusively on the basis of adult morphology. The system proposed by Burckhardt is followed here. With a few exceptions, the illustrations for the present book were prepared by the author. The terminalia of all species (m/f) have been figured. Furthermore, the 5th instar nymphs of nearly all the species have been keyed and figured. For each species, the distribution in and outside Denmark and Fennoscandia is briefly given. Information concerning the Scandinavian countries was compiled from literature and from the author's records based on identifications for museums and private collectors over the last 53 years. Distribution data for outside Fennoscandia and Denmark were derived mainly from Klimaszewski (1973). Information on the biology of the species has been compiled from the literature and from the author's observations.