Numerous yellows-type diseases of plants have been associated with wall-less prokaryote pathogens – phytoplasma over the last 40 years. These pathogens cannot be grown in axenic culture so that advances in their study are mainly achieved by molecular techniques. Severe disease epidemics associated with a phytoplasma presence have been described worldwide. These include coconut lethal yellowing in Africa and the Caribbean, grapevine yellows in major viticultural areas and various diseases affecting stone and pome fruit plants. Phytoplasma-infected plants exhibit symptoms suggesting a profound disturbance in the normal balance of growth regulators and also yellows symptoms, but very often the symptomatology is not diagnostic. Detection and characterization of phytoplasmas infecting different plant species are now possible with molecular methods, based on the study of 16S rDNA polymorphisms. Molecular diversity of phytoplasmas is also demonstrated by studying genes coding the ribosomal proteins S3, tuf, SecY, amp, imp and other genes. Four phytoplasma genomes have been fully sequenced, including those of two ‘Candidatus Phytoplasma asteris’ strains, and those of strains of ‘Ca. P. mali’ and ‘Ca. P. australiense’. Three of these genomes contain large amounts of repeated DNA sequence, and the fourth carries multiple copies of almost 100 genes. Considering that phytoplasmas have unusually small genomes, these repeats might be related to their transkingdom habitat and to their pathogenic activity. An outlook of recent findings in the field is also reported.