‘Candidatus Phytoplasma’ species are mostly transmitted from plant to plant by phloem feeding hemipterans, primarily leafhoppers (Cicadellidae) and planthoppers (Fulgoroidea) (Hemiptera, Auchenorrhyncha). However, there is one group of phytoplasmas, the 16SrX or apple proliferation group, whose members are transmitted by psyllid vectors of the superfamily Psylloidea (Hemiptera, Sternorrhyncha). These psyllid-transmitted phytoplasmas are genetically closely related and are associated with economically important diseases of fruit trees such as pear decline, apple proliferation and European stone fruit yellows. The psyllid vector species of these phytoplasmas are also closely related and all belong to the genus Cacopsylla. Both, phytoplasmas and psyllid vectors, are geographically limited to the Palaearctic region, mainly Europe. Only pear decline and peach yellow leaf roll phytoplasmas have probably been introduced to America along with their vectors. As phytoplasma-infected trees cannot be cured and resistant plant material is not available to the growers, preventive control measures such as vector control are of paramount importance to limit the spread of these diseases. Thus, detailed knowledge about the biology and ecology of the vector species, their host plants as well as knowledge about the transmission parameters is crucial.