The mathematical relationship (breakage equation) between the inlet and outlet particle-size distributions of a roller milling operation is described, and the breakage function linking the two is defined. The forms of the breakage equation and the breakage function are different for roller milling than for other comminution operations, such as hammer milling or ball milling. The breakage equation is discretised to give a matrix form, from which it is demonstrated that during roller milling of wheat, particles break independently of one another. This is an important assumption of breakage equations for many comminution operations and experimental results are presented, which confirm its applicability to the roller milling of wheat grains. Breakage matrices are successfully used to predict the outlet particle-sire distributions from First Break milling of wheat. Later papers in this series consider the form of the breakage function.