II Definitions of Mobility BarriersA. Structural BarriersB. Stigler and the Chicago SchoolC. A Normative Definition of BarriersD. The Value of IncumbencyIII Determinants of the Condition of EntryA. Barriers and Strategic BehaviourB. Scale Economies, Sunk Costs and Limit PricingC. Absolute Cost AdvantagesD. Product DifferentiationE. A Summing-UpIV Exit Barriers and Asset SpecificityV Dynamic Limit Pricing and Models of Sequential CompetitionVI Empirical Models of EntryA. Measuring the Height of Overall BarriersB. The Interaction Between Entry and ProfitsC. Other Models of EntryVII Concluding RemarksReferences