I. Introduction, 327; the measurement of concentration, 328; are most industries concentrated?, 329. — II. Concentration and oligopoly not identical, 331; the importance of conditions of supply, 332; “ease of entry” and “elasticity of supply,” 336; agricultural programs an example of concentration, 339; need for studies of elasticity of supply, 340. — III. Oligopoly may not raise prices, 341; appraising certain empirical evidence, 344. — IV. Summary and conclusion, 345.