Poliovirus is one of the simplest and most extensively studied animal viruses. Yet, in spite of the voluminous literature, many basic and important questions concerning its morphogenesis remain unanswered. For example, what mechanisms insure that the progeny virus retain the precise structural features of the infecting particle ? How does the viral RNA combine with the capsid proteins to form the virion and how are host RNA molecules excluded from this process ? Do non-capsid viral proteins influence these events ? Thus, while infectious tobacco mosaic virus particles have been reconstituted in vitro from viral RNA and protein monomers, both obtained by controlled degradation of the virion, it remains to be shown that this is in fact the mechanism of morphogenesis in vivo.