Porcine reproductive and respiratory syndrome (PRRS) is an infectious disease in swine that emerged 10 years ago. Today, PRRS is endemic in many if not all the pig-producing countries. The syndrome is due to a small enveloped RNA virus which belongs to the new Arteriviridae group. This group also includes the equine arterivirus and the simian hemorrhagic fever virus. The disease produces many clinical symptoms in pigs of any age but the two major features of the syndrome are respiratory and reproductive failure. An influenza-like illness is often observed consisting of a transient loss of appetite, slight hyperthermia and respiratory distress. The reproductive problems include late-term abortion, stillbirth and young piglet mortality. The virus is remarkably well adapted to its natural host. It infects almost exclusively pig monocytes or macrophages. For unexplained reasons, the virus may persist in the body for extended periods (several weeks) after an antibody immune response has been mounted. Within herds, the virus may persist for several years. In the acute phase of the infection, the immune functions of the pig may be impaired, however, a chronic immunosuppressive effect does not seem to occur thereafter. Several vaccines are available, which all have protective value against PRRS clinical disorders. None of them, however, has been tested for its ability to prevent or shorten the persistent infection in individual pigs or the long-term effect of virus circulation within herds. In the future, PRRS virus may be only an additional contaminant of farms with limited effects on profitability; however the possibility for this persistent infection to supply swine or eventually other species with new pathogenic variants must be also considered. This review provides a detailed description of the aetiology, pathology, immunology, diagnostic methods and epidemiology of PRRS.