The nucleus of the eukaryotic cell must carry out many functions simultaneously. These tasks include ensuring that the cell is continuously supplied with an appropriate, changing set of proteins on its way through cell divisions and differentiation. During these processes, the integrity of the genetic material must be maintained against a constant onslaught of damaging physiological and environmental factors. Fulfilling these complex tasks requires the dynamic integration and synchronization of different nuclear functions. Protein modification by ubiquitin is proving to be a crucial tool for nuclear functioning, and is emerging as a decisive mechanism that enables the concerted regulation of nuclear pathways.