Animal cells have a cytoskeleton which, on the one hand, provides stability, structure, form, and polarity. On the other hand, this cytoskeleton is very dynamic and controls cell and organelle movements, shape changes, and even cell division. The cytoskeleton in vertebrate cells consists of three components, comprising microfilaments with a diameter of ca. 7–9 nm, macrofilaments with a dimeter of 25 nm, and intermediate filaments with a diameter of ca. 10 nm, and the latter represents a size in between that of micro- and macrofilaments. All components of the cytoskeleton form networks of fibers inside the cytoplasm. All filaments are formed and degraded by assembly and disassembly of subunits. In case of macro- and microfilaments, these subunits are globular proteins. They have a molecular mass of ca. 50 kDa. In contrast, intermediate filaments are constructed of helical proteins that bundle up into strong fibers (Fig. 4.1). Moreover, in multicellular animals, cells are embedded in tissues and organs. In those structures, cells are connected with each other by cellular junctions, junctions also exist between cells and the extracellular matrix. We discuss tight junctions, adherence junctions, gap junctions, desmosomes and hemidesmosomes. In addition we look at the principal composition of the extracellular matrix with the components collagen, hyaluronic acid, proteoglycans, and anchoring proteins, and discuss the structure and biomineralization of dentin, a major component of our teeth.