I. Perfect monopoly: the short-run marginal cost curve, 174; the long-run cost curve, 179; intraplant vs. interplant economies and diseconomies, 181; the problem of optimum size, 184 — II. Mathematical analysis: geometric proof, 185; algebraic proof, 187. — III. Oligopoly: non-economic considerations, 192; the short-run marginal cost curve, 194; the long-run curve, 196; total cost, 196; long-run equilibrium, 197; elements of instability, 199. — IV. American experience: mixture of types, 200; decline of dominant firms, 202; the milk distributing industry, 203. — V. Conclusions, 205.