After a short introduction on Porcine Reproductive and Respiratory Syndrome (PRRS) regarding the history, the first occurrence in several countries, and the causal virus, designated Lelystad virus, a description is given of the clinical aspects and several diagnostic methods. After some general remarks on the clinical aspects, the epidemic and the endemic phase of the disease are described. Regarding the diagnosis, special attention is paid to the detection of antibodies and of the PRRS Virus (PRRSV). Regarding the detection of antibodies, a description is given of three tests: the immunoperoxidase monolayer assay, the enzyme-linked immunosorbent assay, and the serum neutralization test. Concerning the detection of PRRSV, attention is paid to the isolation of the virus, the demonstration of PRRSV antigens in frozen or fixed tissue using immunohistochemistry or immunofluorescence, the in situ hybridisation technique and the Polymerase Chain Reaction (PCR).