The nucleolus is a nuclear membraneless organelle in which the rDNA transcription takes place an in which the ribosomal subunits are assembled. It is a highly dynamic structure from which the components are dispersed at the onset of mitosis and are reassembled at the end of cell division. The nucleolus can be visualised through electron microscopy while the organization and dynamics can be studied through Fluorescent Protein Tagging and Fluorescent Recovery after Photobleaching (FRAP). Malfunction of nucleoli can be the cause for several human diseases.