The structure of nucleoli differs widely according to their functional state. Main structural components of human and mammalian nucleoli are: the fibrillar centres (FC), the dense fibrillar component (DF), and the granular component (GC). A critical review of the recent literature and of the author's results suggests that in active nucleoli the rRNA gene repeats of the nucleolus organizing regions (NORs) are localized in the DF. The DF contains also enzymes and factors necessary for transcription, RNA processing, and pre-ribosome synthesis. The FC serves possibly as a stock of proteins, among them RNA-polymerase I. The GC is made up mainly of preribosomes. Non active (non transcribed) NORs may be situated far away from nucleoli within the cell nucleus.