Total disc arthroplasty is gaining in popularity as an alternative to spinal fusion. A total disc consists of articulating bearing surfaces with one made from a metal and the other made from either a metal or a polymer. The aim of this study was to determine the contact stresses in lumbar total disc arthroplasty devices. The total disc was modelled as a ball and socket joint and Hertzian contact theory was used to determine the maximum contact stresses. The effect of material combination and implant size on contact stress was investigated. For a typical disc arthroplasty with a ball radius of 14 mm, the contact stresses for metal against polymer and metal against metal material combinations were 3 to 6 MPa and 63-130 MPa, respectively, and were below the fatigue strength of the materials.