Kirzner's (1973) theory of entrepreneurship emphasizes the equilibrating role of entrepreneurship. When the market is not in equilibrium, profit opportunities exist, and entrepreneurs discover and act on these profit opportunities to equilibrate the market. Because Kirzner focuses on entrepreneurial actions when profit opportunities exist, and does not describe where they come from, one could imagine a fixed stock of profit opportunities that get used up as entrepreneurs discover them. But new profit opportunities are being created constantly. A taxonomy of the origins of entrepreneurial opportunities includes factors that disequilibrate the market, factors that enhance production possibilities, and most notably, opportunities created from previous acts of entrepreneurship. Entrepreneurial actions do not use up profit opportunities, but create them, and the critical role of entrepreneurship in the creation of new profit opportunities is emphasized. This line of reasoning leads directly to policy implications regarding the economic environment conducive to entrepreneurial discovery and the role of government in research and development. This paper enhances Kirzner's theory of entrepreneurship by illustrating how entrepreneurship enlarges the stock of future entrepreneurial opportunities, and points to entrepreneurship as the engine of economic progress. Entrepreneurship occurs when an individual acts to take advantage of a profit opportunity that presents itself in the economy. In its simplest form, the entrepreneur might notice that one person is willing to sell something for less than someone else is willing to pay for it, so the entrepreneur can act as a middle man, profiting from buying at the lower price and selling for more. The profit, which is the return to the entrepreneur's alertness to the opportunity, was created entirely by the entrepreneur's activity, because the sale would not have taken place without someone having noticed the profit opportunity. The entrepreneur's activity benefits the buyer, the seller, and more generally, the entire economy. Furthermore, the entrepreneur's profit signals potential suppliers and demanders about their market opportu- nities, and even signals other potential middlemen of the profit opportunity for facilitating exchanges. Eventually the ability to earn above-normal profits will be competed away, but only after those profits have served their role in signaling a way in which resources could be more efficiently allocated in the economy. Entrepreneurship is indispensable for economic progress, but entrepreneurial activity is possible only when profit opportunities are available to the entrepreneur. This paper discusses the ways in which those profit opportunities can arise in an economy.