The existence of a supermolecular structure in the eukaryotic nucleus, involving loops of chromatin attached at irregularly spaced points to the nuclear matrix, is now well established. Quite a bit is known concerning the DNA sequences involved in the attachment. However, the function of this highly organized structure remains largely unknown. In this review, we attempt to provide an overview of present knowledge of nuclear loop structure, and to critically summarize recent studies that provide new insights as to function. We derive the conclusion that the loop structure is very likely one in which DNA topology can be locally regulated by topoisomerase activity, and is most probably modified during development.