‘The nucleus’ considers the most spatially organized and probably the most dynamically active part of the cell. Looking in detail at the structure of the nucleus, the nuclear envelope, pore complexes, the nuclear lamina, the genetic constituents of the nucleus (DNA), nucleoli, Cajal bodies, snurposomes, and spliceosomes are described. The nuclear envelope separates the nuclear contents from the cytoplasm, also controlling a constant and massive molecular interchange between the two compartments. The importance of the discovery of DNA structure is highlighted as an enormous advance in biology. The structure and packaging of DNA and the processes within the nucleus, such as production of ribosomes in the nucleolus are explained.