Phytoplasma associated diseases are spread worldwide, and in several cases are associated with severe epidemic of very often quarantine importance. These plant pathogens are prokaryotes belonging to the Mollicutes class since they lack a cell wall; up to now they were not cultivated in axenic culture therefore Koch postulates are only sometimes fulfilled by using alternative tools, such as graft or insect transmission. The possibility to design specific primers for highly conserved genes such as 16S ribosomal gene together with the use of molecular probes randomly cloned from phytoplasma genome, allowed discriminating and molecularly classifying them. Now a certain amount of knowledge is available that allow starting epidemiological studies in order to prevent further spreading of phytoplasma-associated diseases. In this paper molecular, biological and epidemiological characteristics of phytoplasma associated with important diseases worldwide are described.