Phytoplasmas are bacteria without cell wall belonging to the class Mollicutes. These pathogens are intracellular parasites of plant phloem, transmitted by insects which also serve as their hosts. Phytoplasmas do not cause any disturbance to most insect hosts but their infections of plants is often fatal to this host type. The most common phytplasmoses symptoms in plants are phyllody, virescence, witches broom, reddening or yellowing of leaves, necrosis of the phloem. Knowledge of the biology of these specific organisms is quite limited because they cannot be cultivated in culture medium. Neverheless, in recent time be the complete phytoplasma genome sequencing led to many new insights. Phytoplasmas have an extremely reduced genome (680 to 1600 kb). They lack many essential genes in different metabolic pathways, and most significantly, they can live with the lack of genes for ATP synthase subunits, as well as genes for sugar uptake and usage. It suggests that phytoplasmas effivciently uptake metabolites from the host. Hovewer, many of these uptake mechanisms are including the one for ATP. In spite of everything, phytoplasmas successfully survive and manipulate the hosts from two different kingdoms. It is believed that they have been so well adapted to that way of life because of their genome plasticity. Phytoplasmas compensated their genomic limitations by horizontal gene transfer, rearrangements of DNA and recombination between chromosomes and Potential Mobile Units (PMUs). PMUs contain genes that are important for survival and adaptation to different hosts. For the time being, one of the best ways to gain more knowledge on phytoplasmas is sequencing of additional phytoplasma genomes and comparative genome analysis.