The article contains data on the properties and classification of the etiological agent of porcine eperythrozoonosis (PE). Until the end of the twentieth century PE belonged to the Rickettsiales order and was named Eperythrozoon suis. With the progress of molecular biology this species was reclassified as a member of the Mollicuties class, Mycoplasmatales order, based on similarities of gene sequences and particularly the presence in the genome of the 16S ribosomal RNA gene, but also on other supporting biological transfer properties that it shared with Mycoplasma organisms. It joined the cluster of in vitro uncultivable haemotrophic mycoplasmas (HM) named Mycoplasma suis. PE attaches to the erythrocytes of an infected host where replication and pathogenic activity take place. The main symptoms are haemolytic anaemia, sometimes lethal, primarily in piglets, but also occurring in feeder pigs and pregnant sows. Infected animals can remain apparently normal for months before developing symptoms. It is also possible that many infected animals never show any symptoms. Suckling piglets, but, additionally, also older pigs, often exhibit skin pallor and icterus, which generally results in unthriftiness, poor weight gain and susceptibility for other infectious disease syndromes, caused by facultative pathogens, representing other species of microorganisms. It is stressed that as the result of blood red cell damage by M. suis autoimmune cold agglutinins of IgM class are produced, which play an important role in the pathogenesis of PE. In the following section concerning laboratory diagnostic tests the microscopic diagnosis of blood smears is critically evaluated. As a gold standard the LightCycler real-time PCR assay is indicated. Serological assays as a complement fixation test, indirect haemagglutination test and ELISA were evaluated, indicating that during the time when diagnostic antigens were prepared from the blood of the infected pigs being not sufficiently specific, they delivered unsatisfactory results. The situation improved when for the first time the recombinant expression of p40 and p70 M. suis antigens in Eschericia coli provided diagnostic antigens that are producible and standardized in vitro. Based on these antigens, the highly specific, sensitive and reliable ELISA was established. The following section of the review contains data on prevention and therapy, including reported progress concerning the development of a vaccine against PE.