The notion that the interior of the nucleus is compartmentalized goes back to the discovery of the nucleolus in the 1830s. Today, we know that numerous, discrete domains related to gene expression exist within the interchromatin spaces of the interphase nucleus. These domains might arise from, and thus be positioned by, the transcriptional activity of the chromosomes, themselves tethered to the nuclear envelope, or they might assemble autonomously. Beyond their roles in gene expression or other nuclear functions, the dynamic behaviour of some of these interchromatin domains is providing clues to the modes of mass transport operating in the nucleus, as well as to the long-elusive deep structure of the nucleoplasm.