Disclosed herein is a process and apparatus for grinding tooth surfaces of spur and helical gears by a generating method. The outer tooth surfaces (4) of a theoretical ring gear (6) are conjugate to the grinding surfaces (3) along the width of a generally hourglass shaped grinding wheel (2). The grinding wheel (2) is rotated which in turn causes the theoretical ring gear (6) to rotate. A work gear (8) is rotated in mesh with the internal tooth surfaces (10) of the theoretical ring gear (6) and simultaneously is traversed in mesh across the width of the grinding wheel (2) with the center of the work gear (Xw) moving in a path (12) about the center of the theoretical ring gear (XI).