This paper develops an econometric model of industry dynamics for concentrated markets that can be estimated very quickly from marketlevel panel data on the number of producers and consumers using a nested fixed-point algorithm. Its estimation enables the measurement of economic barriers to entry and the toughness of price competition. The model's econometric error comes from a shock to both potential entrants' sunk costs of entry and incumbents' fixed costs of continuation. We show that the

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... el has an essentially unique symmetric Markov-perfect equilibrium that can be calculated from the fixed points of a finite sequence of lowdimensional contraction mappings. Our nested fixed point procedure extends Rust's (1987) to account for the observable implications of mixed strategies on survival. We illustrate the model's empirical application with ten years of County Business Patterns data from the Motion Picture Theaters in 573 Micropolitan Statistical Areas. Adding four firms to a monopoly market reduces profits per customer by 23 percent. * A replication file will be available at http://research.abbring.org.