The initiation of complex retrovirus assembly involves a recognition event of the genome-length retroviral RNA by the viral Gag protein. The 5' end of the viral genome and at least the NC domain of the Gag polyprotein are involved in this RNA-protein recognition event. Further, structure-function analysis of encapsidation signals and of the Gag polyprotein will help in elucidating the precise interactions. There are many similarities between the simple and complex retroviruses regarding these processes, and investigations of simple retroviruses will greatly aid in defining the underlying mechanisms of these steps with complex retroviruses. However, the complex nature of the relatively large genomic RNA and the Gag polyprotein predict that determination of the precise mechanisms involved in genome recognition, RNA encapsidation and the initiation of virus assembly will provide a formidable challenge to investigators studying these steps in the virus life-cycle.