The nucleolus is the most prominent sub-nuclear structure residing within the nucleus. It is a dynamic, non-membrane bound organelle that is primarily involved in ribosome biogenesis in eukaryotic cells. The typical nucleus contains 2–5 nucleoli, ranging in size from 0.5 to 5 µm in diameter. Nucleoli assemble around tandemly repeated genes for ribosomal RNA (rRNA) forming chromosomal features known as nuclear organizing regions (NORs), which are sites of ribosomal DNA (rDNA) transcription. Three sub-nucleolar compartments can be distinguished in nucleoli: the fibrillar centre (FC), the dense fibrillar component (DFC) and the granular component (GC). The FC is surrounded by the DFC, which is embedded in the GC. The sub-compartments of the nucleolus are associated with the sequential steps in ribosome biogenesis. The FC is enriched in RNA polymerase 1, which is the enzyme involved in the transcription of rDNA. Transcription of rDNA is initiated at the interface between the FC and the DFC. In the DFC, the pre-ribosomal RNA transcripts are cleaved to form 28S, 18S and 5.8S rRNA and modified by small nuclear ribonucleoproteins (snoRNPs). The transcribed RNA is then moved to the GC for further maturation and combined with ribosomal proteins along with 5S rRNA (transcribed in the nucleus by RNA polymerase II) and assembled into the 40s (composed of 18S rRNA + ribosomal proteins) and 60s (composed of 5.8S, 28S and 5S rRNA + ribosomal proteins) ribosomal subunits. These ribosomal subunits are then exported to the cytoplasm giving rise to functional ribosomes capable of mRNA translation. In addition to ribosome biogenesis, the nucleolus is involved in the regulation of mitosis, cell cycle progression, the biogenesis of ribonucleoprotein complexes and the stress response. Furthermore, the nucleolus has been implicated in diseases including viral infections, cancer, progeria and neurodegenerative diseases. In cancer, the size and number of the nucleoli increase, whereas a decrease in the size of nucleoli is associated with neurodegenerative diseases such as Alzheimer’s and Parkinson’s disease. We offer a broad range of nucleolar markers including Fibrillarin antibodies, Nucleolin antibodies, Nucleophosmin antibodies, and NIFK antibodies. We also offer the monoclonal antibody NM95 which recognises an antigenic marker associated with the nucleoli in human cells.