The nucleus is recognized as one of the characteristic features of eukaryotic cells. It comprises the location of the bulk of the cellular genome separated during interphase from the cytoplasm by a pair of bilamellar nuclear membranes. Most of the ultrastructural details of the nucleus have been defined using microscopy, particularly transmission and scanning electron microscopy. These techniques have revealed a great deal of information about nuclear structure, and provide the basis of our conventional conceptual image of the nucleus as an oblate spheroid bounded by paired bilamellar membranes containing grommet-like nuclear pores linking nucleoplasm and cytoplasm. Empirically, employing extraction with non-ionic detergents under high salt conditions, the nucleus can also be divided into two structural components, chromatin and the nuclear matrix. Based on studies on metazoans, the latter comprises nuclear lamina, the nuclear pore complexes, RNP proteins, and nucleoli, and is thought to function as a nucleoskeletal framework within the context of which the various mechanical and enzymatic nuclear activities occur.