Import of viral DNA into the nucleus is essential for the successful replication of DNA tumour viruses. To achieve this goal, viruses have adapted strategies to traverse the barriers between the plasma membrane and the nucleus of a host cell. Two DNA tumour viruses, simian virus 40 and adenovirus, achieve the nuclear-entry step in slightly different ways. SV40 DNA enters the nucleus through the nuclear pore complexes (NPCs) in apparently intact virions. By contrast, adenovirus particles dissociate near the NPC before the viral DNA is imported into the nucleus. In both cases, karyophilic protein components of the viruses appear to mediate nuclear entry of the viral genomes. In this article, we discuss how an understanding of the cell biology of virus entry can help us understand the process of nuclear transport.