The motion of balls deep within the charge of a rotary grinding mill was investigated, using novel techniques to track the balls. These included X-ray filming of both the front and side views of a transparent mill, and gamma-camera filming of a radioactive ball. Analysis of the trajectories of the balls revealed several phenomena in the milling action that have hitherto not been reported in the literature. These include: the non-rotation of the balls which indicates that the grinding does not take place by rotary nipping of the ore between the balls; charge dilation with increased mill speed; longitudinal migration of the balls; further insights into charge segregation; spiralling action of the balls; smooth paths of balls in the bulk of the charge.