Phytoplasmas are intracellular bacteria that lack a cell wall and are limited to phloem tissues, affecting a wide range of plant hosts, and causing a variety of symptoms and devastating infections. Replicating only in the body of vector insects and in host plant cells, these organisms have a unique biology. This chapter summarizes the discovery of phytoplasmas, taxonomy, their current status, diagnosis, epidemiology, transmission and spread, economic losses, and potential control methods.