Hepatitis delta virus (HDV) contains a circular, viroid-like RNA genome, the only animal viral RNA of its kind. It possesses a ribozyme activity, which can autocatalytically cleave and ligate itself. The ribozyme has a unique structural requirement different from other known ribozymes. HDV RNA undergoes RNA-dependent RNA replication via a double rolling circle mechanism, which is probably mediated by cellular RNA polymerase II, utilizing modified cellular transcription machineries. HDV RNA encodes a single protein, hepatitis delta antigen, which is a nuclear, RNA-binding phosphoprotein and required for viral RNA replication. During replication, HDV RNA undergoes a specific RNA editing event to extend its open reading frame and produce a longer, isoprenylated delta antigen, which suppresses RNA replication and initiates viral particle assembly. Ribozyme, cell-mediated RNA-dependent RNA replication, and RNA editing are some of the unique properties and unresolved issues of the molecular biology of HDV.