Two main principles of nuclear organization have been outlined on the basis of contributions by many research groups in recent years. The first principle is that interphase chromosomes occupy discrete territories in the nucleus, with no intermingling of the DNA from different chromosomes. Within a chromosome territory the DNA is organized in chromatin fibers at several levels of folding, that meander through the territory. Transcription and replication take place at the surface of these higher order chromatin fibers, probably on locally unfolded DNA templates. The second principle is that different types of nuclear domains are associated with several specific gene loci. This holds for clusters of interchromatin granules, coiled bodies, RNA 3'-cleavage factor-containing nuclear bodies (cleavage bodies) and probably PML-containing nuclear bodies. These domains may play an important role in the spatial arrangement of genes in the interphase nucleus. Despite these new insights, our knowledge of the function of many nuclear compartments and the molecular interactions responsible for the dynamic organization of a compartmentalized nucleus is still in its infancy.