Classical swine fever is one of the most important viral diseases of pigs and a major source of loss for pig producers worldwide. The disease was eradicated from Great Britain in 1966 and, apart from a single incident in 1971, has not reappeared. A major epizootic started in Europe in 1982-83 and continued through 1984. Extra vigilance is necessary in order to detect any introduction of infection. The disease takes several forms apart from the dramatic disease described in textbooks. These include mild and chronic disease and reproductive losses of various types. Mild disease is a feature of the epizootic in Europe at present. Diagnostic methods are outlined. Diagnosis is frequently difficult and requires laboratory help. The selection and handling of specimens for the laboratory is critical to the success of diagnosis.