Underfunding jeopardizes the ability of protected areas to safeguard biodiversity and the benefits that intact nature provides to society. In this article, we evaluate the cost of effectively managing all existing protected areas in developing countries, as well as the cost of expansion into high-priority new areas. We find that recent studies converge on a funding shortfall of $1 billion to $1.7 billion per year to manage all existing areas. The costs of establishing and managing an expanded protected-area system would total at least $4 billion per year over the next decade, an amount that far exceeds current spending but is well within the reach of the international community. These findings indicate the need for rapid action to mobilize significant new resources for the developing world's protected areas. In particular, this will require (a) the use of a range of tools to generate funds and improve efficiency of management; (b) greater precision and better communication of the costs and benefits of protected areas, both locally and globally; and (c) increased, stable support from developed countries for on-the-ground management of protected-area systems in developing countries.