The transport of proteins between the nucleus and cytoplasm occurs through nuclear pore complexes and is facilitated by numerous transport factors. These transport processes are often regulated by post-translational modification or, reciprocally, transport can function to control post-translational modifications through regulated transport of key modifying enzymes. This interplay extends to relationships between nucleocytoplasmic transport and SUMO-dependent pathways. Examples of protein sumoylation inhibiting or stimulating nucleocytoplasmic transport have been documented, both through its effects on the physical properties of cargo molecules and by directly regulating the functions of components of the nuclear transport machinery. Conversely, the nuclear transport machinery regulates the localization of target proteins and enzymes controlling dynamics of sumoylation and desumoylation thereby affecting the sumoylation state of target proteins. These inter-relationships between SUMO and the nucleocytoplasmic transport machinery, and the varied ways in which they occur, are discussed.