Psyllids have shifted lately into general awareness as vectors of pathogens causing serious plant diseases, as pests in agriculture and forestry or as potential control agents of alien invasive weeds. These small insects are plant sap feeders which are generally very host specific. In addition, related psyllid species tend to develop on related plant species. This makes them an ideal group for studies on insect–plant cospeciation. A sound taxonomic and phylogenetic base is a prerequisite for successful pest control and meaningful research on insect–plant interactions. Currently almost 4000 named species of Psylloidea exist worldwide of which half was described in the last three decades. Despite this tremendous progress there are at least another 4000 species which remain undescribed particularly in Africa, South America and tropical Asia. Since White and Hodkinson’s seminal paper in 1985 a series of studies tested their classification with additional taxa using morphological and molecular techniques. In 2012 Burckhardt and Ouvrard proposed a new classification. Five of their families (Calophyidae, Carsidaridae, Homotomidae, Phacopteronidae and Triozidae) are identical with or similar to those of White and Hodkinson but three differ fundamentally from previous classifications (Aphalaridae, Liviidae and Psyllidae). Many of the recognised families and subfamilies are restricted more or less exclusively to a single plant taxon, e.g. Calophyidae, Phacopteronidae and Rhinocolinae to Sapindales, Carsidaridae to Malvaceae, Homotomidae to Moraceae, Spondyliaspidinae to Myrtaceae, and many Psyllidae to Fabaceae. The 1000 described species of Triozidae, in contrast, have colonised many families of dicots and, a few, even monocots and conifers. Several analyses suggest that cospeciation may be less important than geographical vicariance to explain the observed species richness in psyllids, and that shifts to new host taxa are frequent.