The nucleolus is the nuclear organelle for ribosomal DNA transcription, rRNA processing, and ribosome synthesis. However, abundant recent findings also have shown that the nucleolus is the site where virtually all eukaryotic RNA polymerase-transcribed RNAs are processed and where other important cellular functions are regulated. Although the number of nucleoli per cell and the nucleolar ultrastructures within animal and plant nucleoli differ, most nucleolar functions for those two organisms appear to be conserved. This review not only presents molecular details already known from yeast and animal systems, but also discusses recently published results about nucleolar functioning in plants, a research area still in its infancy.