Genome-reduced, wall-less, and fastidious bacteria of the genera Spiroplasma, Mycoplasma, Phytoplasma and allies belonging to the class Mollicutes, are known for a number of unique microbiological features, which have prompted researchers to investigate their basic, applied, and medical aspects. They are mostly parasitic or symbiotic to a variety of animals and plants, living on or within the eukaryotic cells. Spiroplasmas, recognized by their characteristic spiral shape and active twitching motility, are associated with insects and/or plants. S. poulsonii causes remarkable reproductive phenotype, called male-killing, of their insect hosts. S. citri and S. kunkelii are notorious as devastating pathogens of citrus and maize, respectively. Mycoplasmas are not only medically important as human and animal pathogens like M. pneumoniae and M. genitalium, but also intensively investigated as minimal-genome bacterial models. Microbial genome synthesis and engineering technologies have been developed mainly on M. mycoides and M. capricolum. Some mycoplasmas are known for their capability of unique gliding motility. Phytoplasmas are obligatorily parasitic to plant phloem tissues and vectored by plant-sucking insects, often causing spectacular plant phenotypes like phyllody, virescence, witch’s broom, etc.