This paper extends the static analysis of oligopoly structure into an in nite-horizon setting with
sunk costs and demand uncertainty. The observation that exit rates decline with rm age motivates the
assumption of last-in rst-out dynamics: An entrant expects to produce no longer than any incumbent.
This selects an essentially unique Markov-perfect equilibrium. With mild restrictions on the demand
shocks, sequences of thresholds describe rms' equilibrium entry and survival decisions. Bresnahan and
Reiss's (1993) empirical analysis of oligopolists' entry and exit assumes that such thresholds govern
the evolution of the number of competitors. Our analysis provides an in nite-horizon game-theoretic
foundation for that structure.