Data obtained from a wide variety of test methods both in vitro and in vivo and histological studies of stressed skin have led to an understanding of the mechanical properties of the dermis and the relation of these properties to the structure of the collagen and elastin fiber networks of the dermis. The mechanical properties are found to be well adapted to the mechanical function of the dermis. The viscoelastic nature of the skin shows that a simple structural model based only on collagen and elastin is not adequate for a full understanding of this tissue. The role of proteoglycans in bonding collagen fibrils into large fibers and in connecting these fibers into the fibrous net of the dermis is not well understood. Mechanical testing can yield information on the nature of bonds at this level of structure.