This chapter reviews some important properties of collagen-rich tissues. Collagen is a multifunctional family of proteins of unique structural characteristics. The collagen-rich tissues can be thought of as a composite polymeric material in which the highly oriented crystalline collagen fibrils are embedded in the amorphous ground substance of noncollagenous polysaccharides, glycoproteins, and elastin. The stress–strain curves of a collagenous tissue such as tendon exhibit nonlinear behavior. Cartilage is another collagen-rich tissue which has two main physiological functions. One is the maintenance of shape, and the other is to provide bearing surfaces at joints. Material degradation can result from biological processes such as enzymatic degradation or environmentally induced degradation from mechanical, metal-catalyzed oxidation, and from the permeation of body fluids into the polymeric devices. The isolation and purification of soluble collagen molecules from a collagenous tissue is achieved by using a proteolytic enzyme such as pepsin to cleave the telopeptides.