The incentive to "overinvest" in capital may be eroded in dynamic, competitive settings if firms face uncertainty and irreversibility. In this paper, we derive the stationary Markov perfect equilibrium for a dynamic, infinite-horizon capacity investment game formulated in continuous time in which reduced-form profits are subject to industry shocks. We show that the theory of marginal Tobin's q still holds in capital-accumulation games under uncertainty if the strategic externalities of rivals' investment are properly accounted for.