This paper argues that perceptual distortions and prevailing economic rationality, far from encouraging investment in natural capital, actually accelerate the depletion of natural capital stocks. Moreover, conventional monetary analyses cannot detect the problem. This paper therefore makes the case for direct biophysical measurement of relevant stocks and flows, and uses for this purpose the ecological footprint concept. To develop the argument, the paper elaborates the natural capital concept and asserts the need of investing in natural capital to compensate for net losses. It shows how the ecological footprint can be used as a biophysical measure for such capital, and applies this concept as an analytical tool for examining the barriers to investing in natural capital. It picks four issues from a rough taxonomy of barriers and discusses them from an ecological footprint perspective: it shows why marginal prices cannot reflect ecological necessities; how interregional risk pooling encourages resource liquidation; how present terms of trade undermine both local and global ecological stability; and how efficiency strategies may actually accelerate resource throughput. Affirming the necessity of biophysical approaches for exploring the sustainability implications of basic ecological and thermodynamic principles, it draws lessons for current development.