The substitution of tissues due to tumors, pathologies, or traumatic accidents is well known, and different surgical branches perform these kinds of surgeries on a daily basis with relative ease. The substitution of bone tissue is especially relevant as it contributes to the structural stability of the body. Bone tissue is composed of an organic matrix, a mineral component, and water in approximately similar volumes. The combination of these elements forms a composite material with different hierarchical levels in its microstructure. The reproduction of this highly hierarchical structure is still not possible. Historically, articular prostheses have been made of metallic materials due to the high mechanical tensions that they suffer once implanted, as well as good fatigue resistance and tenacity. Lately, the use of polymeric materials has attracted a great deal of attention. On the other hand, the choice of a material for the filling of bone defects or bone cavities is greater, ranging from biodegradable polymers to calcium phosphate cements and ceramics. The role of polymers in bone substitution is relevant but limited to a few applications, such as articulating bearing surfaces of joint replacements, both hip and knee, and as interpositional cementing material between the implant surface and the bone. In the first application, the ultimate choice is ultrahigh molecular weight polyethylene, and in the second application the most used polymer is poly(methyl methacrylate). The use of biodegradable polymers has grown significantly in applications dealing with support structures needed for normal movements of articulating joints. In this chapter, the most used polymers in orthopedic applications will be described. Special emphasis will be given to their physical and chemical properties.