Fish herpesviruses can cause significant economic losses in aquaculture, and some of these viruses are oncogenic. The virion morphology and genome organization of fish herpesviruses are generally similar to those of higher vertebrates, but the phylogenetic connections between herpesvirus families are tenuous. In accordance with new taxonomy, fish herpesviruses belong to the family Alloherpesviridae in the order Herpesvirales. Fish herpesviruses can induce diseases ranging from mild, inapparent infections to serious ones that cause mass mortality. The aim of this work was to summarize the present knowledge about fish herpesvirus diseases. ACTA VET. BRNO 2012, 81: 383–389. Agnieszka Lepa, Andrzej Krzysztof Siwicki. Inland Fisheries Institute, Department of Fish Pathology and Immunology, Olsztyn, Poland. Phone: +48 227562490. E-mail: alepa@infish.com.pl