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We derive a closed-form solution for Tobin's Q in a stochastic dynamic framework. We show analytically that investment is positively related to Tobin's Q and cash flow, even in the absence of adjustment costs or financing frictions. Both Q and investment move in the same direction as expected revenue growth, so changes in expected revenue growth induce Q and investment to comove positively. Similarly, shocks to current cash flow, arising from shocks to the user cost of capital in our model, cause investment and cash flow per unit of capital to comove positively. Furthermore, we show that this alternative mechanism for the relationship among investment, Q, and cash flow delivers larger cash flow effects for smaller- and faster-growing firms, as observed in the data. Moreover, the empirically small sensitivity of investment to Tobin's Q does not imply implausibly large adjustment costs in our model (since there are no adjustment costs). Calibrating the model generates values of Q similar to those in the data; investment is more sensitive to cash flow than it is to Q, and both responses are of empirically plausible magnitudes. Copyright 2011, Oxford University Press.

Investment is characterized by costly reversibility when a firm can purchase capital at a given price and sell capital at a lower price. We derive an explicit analytic solution for optimal investment by a firm facing costly reversibility. In addition, we derive a local approximation to the solution which highlights the effects of the parameters of the problem on the triggers for investment. More generally, we extend the Jorgensonian concept of the user cost of capital to the case of uncertainty and define cU and cL as the user costs of capital associated with the purchase and sale of capital, respectively. Optimality requires the" firm to purchase and sell capital as needed to keep the marginal revenue product of capital in" the closed interval [cU,cL]. This prescription encompasses the case of irreversible investment as well as the standard" neoclassical case of costlessly reversible investment. Finally, quantitative analysis suggests" that even when the difference between the purchase and sale prices of capital is small user costs associated with purchasing and selling capital are closer to those applicable under" complete irreversibility than to those applicable under costless reversibility."