Since its discovery nearly 30 years ago, hepatitis delta virus (HDV) has continued to surprise and fascinate. At 1,680 nucleotides the HDV genome is the smallest known to infect man. It is unique among animal viruses, the closest known relatives being plant viroids. To compensate for its limited protein coding capacity, HDV relies heavily on host functions and on structural features of its circular RNA genome. HDV infection depends on hepatitis B virus as a helper, and increases the severity of liver disease caused by HBV alone. There is currently neither an effective HDV vaccine nor a generally accepted useful therapy for HDV infection. This volume encompasses recent developments in HDV research, from molecular virology to genetics to experimental investigation of new therapeutic and vaccine candidates.