One of the most intriguing aspects of hepatitis delta virus (HDV) is that, even though it is a naturally occurring infectious agent of human beings, it nevertheless shows a number of remarkable similarities to viroids, the subviral agents of plants that are considered elsewhere in this volume. The recognition and exploitation of this analogy has been a major contributor to studies of HDV genome replication. This review will concentrate on the structure of the HDV genome and the mechanism by which the genome is replicated, with special emphasis on similarities to the viroids. The review will not consider in any depth other important aspects of HDV, such as the assembly of genomes into virus particles, experimental transmission, host responses, or epidemiology. For such information the reader is directed to other more extensive reviews (Lai 1995; Purcell and Gerin 1996; Hadziyannis 1997; Margolis et al. 1997). However, to put this review in perspective, we will present a limited amount of background material.