A comparison is made between the lubrication mechanisms believed to occur in normal human hip joints, and artificial hip joints. A review of the literature and some new experimental data is used to highlight many of the characteristics of human joint lubrication. Specimens of artificial joints removed from patients after various periods of service were studied, and deductions made about the modes of lubrication and wear. Both metal-on-metal and metal-on-plastic joints were considered. It was concluded that human joints operated mainly by fluid film lubrication while artificial joints displayed principally boundary lubrication.