We estimate investment policy functions under general assumptions about technology and markets. Policy functions are easy to estimate and summarize the key predictions of any dynamic investment model. Because our method does not rely on Tobin's Q, it does not require information about market values and can be readily applied to study private firms. In addition, unlike Tobin's Q, we show that investment policy functions account for a large fraction of the variation in corporate investment. As such they are much better suited to evaluate and estimate dynamic investment models. Using this superior characterization of firm investment behaviour we then use indirect inference methods to estimate deep parameters of a structural model of investment featuring decreasing returns to scale and generalized adjustment cost functions.