This chapter discusses the architecture of tissues from a mechanical point of view. It also discusses the cell junctions that link cells together in the flexible, mobile tissues of animals. The tissues of an adult animal are maintained and renewed by the same basic processes that generated them in the embryo: cell proliferation, cell movement, and cell differentiation. As in the embryo, these processes are controlled by intercellular communication, selective cell-cell adhesion, and cell memory. When plant cells become specialized, they generally produce specially adapted types of walls: waxy, waterproof walls for the surface epidermal cells of a leaf; hard, thick, woody walls for the xylem cells of the stem; and so on. Cancers arise from the accumulation of many mutations in a single somatic cell lineage; they are genetically unstable, having increased mutation rates and, often, chromosomal abnormalities. In many tissues, nondividing, terminally differentiated cells are generated from stem cells, usually via the proliferation of precursor cells.