In eukaryotic cells, the physical separation of the genetic material in the nucleus from the translation and signaling machinery in the cytoplasm by the nuclear envelope creates a requirement for a mechanism through which macromolecules can enter or exit the nucleus as necessary. Nucleocytoplasmic transport involves the specific recognition of cargo molecules by transport receptors in one compartment followed by the physical relocation of that cargo into the other compartment through regulated pores that perforate the nuclear envelope. The recognition of protein cargoes by their transport receptors occurs via amino acid sequences in cargo proteins called nuclear targeting signals. Both nuclear import and export of proteins are highly regulated processes that control, not only what cargo can enter and/or exit the nucleus, but also when in the cell cycle and in what cell type, the cargo can be transported. Deregulation of the nuclear transport of specific cargoes has been linked to numerous cancers and developmental disorders highlighting the importance of understanding the mechanisms underlying nucleocytoplasmic transport and particularly the modulation of the specific interactions between transporter receptors and nuclear targeting signals within target cargo proteins.