This paper presents a methodology of a quantitative characterization of the kinematics of evolution of a damage zone surrounding the tip of a slowly propagating crack. On the basis of the Crack Layer Theory, the evolution of the damage zone is modelled as a combination of a few elementary motions: translation, rotation, isotropic expansion and distortion. A procedure for evaluating the rates of the elementary motions on the basis of direct measurements is developed. The procedure is illustrated for curved crack layer growth in the vicinity of a hole in commercial polystyrene. The important role of the damage zone in determining the main crack trajectory and speed is clearly demonstrated.