A new disease, the porcine reproductive and respiratory syndrome (PRRS) has emerged in the eighties in North-America and Europe. This disease consisting mainly in late abortions, fœtal mortality and newborn deaths is due to a small enveloped virus which is close to the other known Arteriviruses which affect the mouse, the horse or the monkey. Arteriviruses are positive single strand RNA viruses. Virus gene expression requires the formation of sub-genomic mRNA. These mRNA are polycistronic in their organisation but they are functionnally monocistronic. Only the open reading frame at the 5’ extremity is expressed. Two genotypes of porcine Arterivirus are encountered. The north-american strains are genetically distinct from the european strains. They probably have evolved independently from a common virus ancestor. The structure and molecular biology of this virus, and its replication strategy as well as its interaction with its host, are developed in this paper.