Microeconomic theories of industry evolution reserve key roles for the entry and exit of firms. Competitive models of industry supply often assume that individual firms have Ushaped long run average cost curves; in consequence, an industry's output adjustment in movement from one long run equilibrium to another is entirely accomplished through the entry and exit of firms, rather than through changes in long-run equilibrium output of individual firms. In empirical applications, the speed of movement to new equilibrium levels of output and price (measured by the time it takes for profits to return to competitive levels following a demand shock) clearly depends upon the ease of entry and exit.' However, the simplest competitive models have a clear theoretical weakness: abundant evidence against U-shaped long run average cost curves precludes a simple identification of entry with industry expansion and exit with industry contraction. Expansion of industry supply can be met either by incumbents or entrants, and we have little empirical evidence of the division. Entry conditions are also a staple of oligopoly and monopoly models. Potential entry constrains the pricing options open to industry leaders and actual entry (of new firms and new products) erodes, at varying rates, the market power of incumbent firms. If actual entry responds rapidly to profit opportunities, then the persistent exercise of monopoly power will be rare. However, analyses of monopoly and oligopoly models typically approach entry indirectly, by inferring that persistent profit differentials reflect high barriers to entry. There are few direct tests of the effects of such barriers on actual entry. Despite their importance, very few empirical studies of entry and exit exist. Severe data limitations restrict both the number of studies and the scope of extant analyses. The research reported in this paper provides measures of entry and exit in 46 food manufacturing industries during the 1976-82 period. Measures of incidence (the number of firms), as well as extent (the size of firms), are provided. The analysis also distinguishes between diversified firms and specialist producers. Entry and exit rates vary considerably across industries, and the paper reports the results of statistical analyses that account for industry differences in entry and exit rates by specialist firms.