IN THE open treatment of fractures, plates and screws are the devices most commonly used for internal fixation. The successful use of this method of fixation depends on fundamental mechanical principles which will be reviewed in this discussion. The indications, surgical technique, and postoperative care will not be considered at this time. The relative merits of other methods of fixation will also be omitted except to say that wire, pins, intramedullary devices, and bone grafts have their proper place, and in many instances the use of one or more of these is the method of choice. Failures of internal fixation, which are far too common, are often attributed to factors such as metal, electrolysis, absorption, or pressure necrosis. These terms are often meaningless or incorrect as explanations for failures, which can be attributed to more obvious and direct causes. The fault usually can be found in defective materials or in