We propose a new method to estimate the marginal value of capital under minimal assumptions. By combining asset prices with fundamentals, our method provides a quasi‐model‐free marginal q together with a simple correction for measurement error in (average) Tobin's Q using linear regressions. Marginal q yields plausible and robust estimates of adjustment costs and investment sensitivities to fundamentals. The widening gap between marginal q and Tobin's Q is driven primarily by market power and intangible capital. Our novel findings strongly support the neoclassical theory of investment and challenge the widespread use of Tobin's Q as proxy for investment opportunities.