This report presents a review and extension of current grinding theory. Grinding operations are classified on the basis of (a) the cutting action of the grit, (b) the geometry of wheel and work, (c) the type of chip produced, (d) the existence or otherwise of a continuous lateral traverse of the wheel. The theory is developed using the above method of classification. Recent experimental work by Purcell(12) on the wear of the wheel in surface grinding is reinterpreted in the light of the theory. It is shown to be consistent with the postulate of a constant task per grit. A method of calculation is suggested by which a satisfactory surface grinding technique may be applied to the form grinding of gears, where the task per grit is not independent of the imposed grinding conditions.