The material properties of articular cartilage are presented, with particular emphasis on understanding the role of collagen, proteoglycan, and water in providing these mechanical properties. Descriptions of cartilage de-formational behavior and data on its permeability, compressive, tensile, shear, and swelling properties are summarized, as well as a discussion to show why these material characteristics are important for the normal function of the knee. In addition, we provide adescriptiveexplanation of the biphasic theory required to understanding the mechanical behavior of any hydrated soft tissue.