The term scapulohumeral rhythm is commonly used to describe the two dimensional rotation of the scapula accompanying motion of the arm. Despite the development of a variety of measurement techniques, including radiography, goniometry and three dimensional digitisation, the complete three dimensional kinematics of the scapula have never been presented. Nor have the effects of arm motions outside elevation in the coronal, sagittal or scapula planes been considered. Employing the Isotrak®" electromagnetic measurement system, this study has developed and validated a new method to simultaneously measure the three dimensional kinematics of the scapula and humerus. Euler angle rotations of the· scapu·lawere defined in a sequence approximately analogous to clinical definitions. For the first time, the three dimensional displacements of the scapula have also been determined. 950/0 confidence intervals for lateral rotation of the scapula during humeral elevation in the coronal plane have been calculated at under 4°, Significantly smaller than those presented by previous authors. A mathematical model of three dimensional scapulohumeral kinematics has been developed, capable of predicting the position and orientation of the scapula for a given orientation of the humerus over a wide range of humeral motion. Using this model system, the effects of humeral azimuth, elevation and rotation on the kinematics of the scapula have been investigated. Humeral elevation was seen to have the largest effect, causing the scapula to rotate laterally, retract and tip backwards. Humeral azimuth. has no noticeable effect on the lateral rotation of the scapula, although it causes the scapula to retract, and to tip backwards slightly. Rotation of the humerus has littre effect on the kinematics of the scapula. However, when approaching maximal internal rotation, the ligaments around the glenohumeral joint impose a kinematic constraint on the scapula, resulting in elevation of the scapula upon the thoracic cage.