Many viruses replicate in the nucleus of their host cells. To gain access to this compartment, they must navigate their way from the cell surface, through the endosomal or plasma membrane, across a crowded cytoplasm and finally cross the nuclear envelope. Entry into the nucleus typically occurs via nuclear pores, which have a strict size limit that most viruses cannot traverse without prior uncoating or disassembly. While the majority of the viruses make use of the cellular nuclear import machinery for their nuclear transport, a growing number of viruses seem to use specialized, virus-encoded routes of nuclear import. For nuclear-replicating viruses, entry into the nucleus is highly dependent on prior trafficking and uncoating events that act to prime the virus genome for its final destination.