Tomography is the only technique that gives a precise representation of the temporomandibular articulation. To delineate a discrete impingement upon the articular space and compare before-and-after treatment modalities it is essential to strictly respect the angular criteria. Selective tomography can be accomplished only if three criteria are met: 1. To make an axial cephalogram perpendicular to the submental-vertical film and to measure the angle of the grand axis of each condyle with the median sagittal plane; 2. To orient the incident ray perpendicular to this axis for the sagittal sections and parallel for the frontal sections; 3. To be sure of the exact position of the head on the tomograph. The teeth also must be placed in maximum occlusal contact.