An experimental study of the kinematics of the gravity flow of granular materials through hoppers is presented. A radiographic technique was employed to determine simultaneously the velocity and porosity fields in a granular material flowing through a model hopper. It was found that the flow of granular materials through hoppers is characterized by regions of rigid body motion interspersed with narrow bands of drastic change in void ratio and velocity. A major conclusion of this study is that the kinematic assumptions employed in previous theoretical analyses of granular materials flowing through hoppers or in converging channels are not consistent with the salient kinematic phenomena observed. Knowledge of the kinematic phenomena detailed here is basic to the proper analysis and design of hoppers.