Publisher Summary Together with flaviviruses and hepatitis C virus (HCV), an important human pathogen, the pestiviruses are classified as a genus within the family Flaviviridae. Viruses in this family are small, enveloped, positive-strand RNA viruses. Compared with HCV, the pestivirus genome encodes two additional proteins, an N-terminal autoprotease, Npro, and the envelope protein Erns. Comparison of the amino acid sequences of Erns of pestiviruses with amino acid sequences in databases identified two short stretches, both eight amino acids in length, that are homologous to the active site domains of ribonucleases of the RNase T2 family. Enzymatic tests of purified Erns proved that these stretches are involved in ribonuclease activity. This chapter reviews the specific properties of this RNase activity and its function in relation to the life cycle of pestiviruses. The properties of the RNase activity of CSFV Erns were determined with purified native Erns, and Erns purified from insect cells. For ribonucleases of the RNase T2 family, it has been shown that the histidine residues in the two conserved domains are essential for RNase catalysis. Mutational studies showed that this is also true for Erns. Besides the possible cytotoxic action of Erns toward the host immune system there is also a possible role for Erns RNase activity in regulation of RNA synthesis in infected cells.