Mortalities in a number of commercially important mollusc species have been associated with the detection of viruses belonging to several families. The first description of a virus was in adult eastern oyters, Crassostrea virginica, with the detection of virus particles resembling members of the family Herpesviridae. Subsequently, mass mortalities in French stocks of adult Portuguese oysters, C. angulata, were associated with irido-like virus infection. Other viruses observed in molluscs are described as members of the families Iridoviridae, Papovaviridae and Reoviridae. Little information is available on viral infections that affect molluscs due to primarily to the inadequacy of diagnostic methods that are employed when mass mortality events occur. Most laboratories involved in mollusc pathology still employ histopathology for the analysis of samples. Ostreid herpesvirus 1 (OsHV-1) is the only well characterised shellfish pathogen with complete genome sequence now available.