Phytoplasmas are a large group of plant-pathogenic wall-less, non-helical, bacteria associated with diseases, collectively referred to as yellows diseases, in more than a thousand plant species worldwide. Many of these diseases are of great economic importance. Phytoplasmas are difficult to study, in particular because all attempts at culturing these plant pathogens under axenic conditions have failed. With the introduction of molecular methods into phytoplasmology about two decades ago, the genetic diversity of phytoplasmas could be elucidated and a system for their taxonomic classification based on phylogenetic traits established. In addition, a wealth of information was generated on phytoplasma ecology and genomics, phytoplasma–plant host interactions and phytoplasma–insect vector relationships. Taxonomically, phytoplasmas are placed in the class Mollicutes, closely related to acholeplasmas, and are currently classified within the provisional genus ‘Candidatus Phytoplasma’ based primarily on 16S rDNA sequence analysis. Phytoplasmas are characterised by a small genome. The sizes vary considerably, ranging from 530 to 1350 kilobases (kb), with overlapping values between the various taxonomic groups and subgroups, resembling in this respect the culturable mollicutes. The smallest chromosome, about 530 kb, is known to occur in the Bermuda grass white leaf agent ‘Ca. Phytoplasma cynodontis’. This value represents the smallest mollicute chromosome reported to date. In diseased plants, phytoplasmas reside almost exclusively in the phloem sieve tube elements and are transmitted from plant to plant by phloem-feeding homopteran insects, mainly leafhoppers and planthoppers, and less frequently psyllids. Most of the phytoplasma host plants are angiosperms in which a wide range of specific and non-specific symptoms are induced. Phytoplasmas have a unique and complex life cycle that involves colonisation of different environments, the plant phloem and various organs of the insect vectors. Furthermore, many phytoplasmas have an extremely wide plant host range. The dynamic architecture of phytoplasma genomes, due to the occurrence of repetitive elements of various types, may account for variation in their genome size and adaptation of phytoplasmas to the diverse environments of their plant and insect hosts. The availability of five complete phytoplasma genome sequences has made it possible to identify a considerable number of genes that are likely to play major roles in phytoplasma–host interactions. Among these, there are genes encoding surface membrane proteins and effector proteins. Also, it has been shown that phytoplasmas dramatically alter their gene expression upon switching between plant and insect hosts.