Israel Kirzner's concept of entrepreneurship as alertness to profit opportunities is one of the most influential modern interpretations of the entrepreneurial function. Shane and Venkataraman's (2000: 218) influential assessment defines entrepreneurship research as "the scholarly examination of how, by whom, and with what effects opportunities to create future goods and services are discovered, evaluated, and exploited." As such, "the field involves the study of sources of opportunities; the processes of discovery, evaluation, and exploitation of opportunities; and the set of individuals who discover, evaluate, and exploit them." Shane's General Theory of Entrepreneurship (2003) cites Kirzner and "Kirznerian opportunities" more than any writer other than Joseph Schumpeter. More generally, the entrepreneurial opportunity, rather than the individual entrepreneur, the startup company, or the new product, has become the centerpiece of the academic study of entrepreneurship. In Kirzner's framework, profit opportunities result from prices, quantities, and qualities that diverge from their equilibrium values. Some individuals tend to notice, or be alert to, these opportunities, and their actions bring about changes in prices, quantities, and qualities. The simplest case of alertness is that of the arbitrageur, who discovers a discrepancy in present prices that can be exploited for financial gain. In a more typical case, the entrepreneur is alert to a new product or a superior production process and steps in to fill this market gap before others. Success, in this view, comes not from following a well-specified maximization problem, such as a search algorithm (High, 1980), but from having some insight that no one else has, a process that cannot be modeled as an optimization problem. Entrepreneurship, in other words, is the act of grasping and responding to profit opportunities that exist in an imperfect world. Unlike other approaches in modern economics, the imperfections in question are not seen as temporary "frictions" resulting from ill-defined property rights, transaction costs, or asymmetric information. While those imperfections can be cast in an equilibrium mold - as in the modern economics of information-Kirzner has in mind a market in permanent and ineradicable disequilibrium. Kirzner's approach, like that of Knight, Schumpeter, and other key contributors to the economic theory of entrepreneurship, sees entrepreneurship as an economic function, not an employment category (i.e., self-employment) or type of firm (i.e., a startup company). The main effect of the entrepreneurial function is market equilibration: by closing pockets of ignorance in the market, entrepreneurship always stimulates a tendency towards equilibrium (Selgin, 1987). While Kirzner's "pure entrepreneur," an ideal type, performs only this function, and does not supply labor or own capital, real-world business people may be partly entrepreneurs in this sense, partly laborers, partly capitalists, and so on. The relationship between entrepreneurial discovery and capital investment distinguishes Kirzner's approach sharply from Knight's (and, arguably, from Kirzner's mentor Ludwig von Mises's). Because Kirzner's (pure) entrepreneurs perform only a discovery function, rather than an investment function, they do not own capital; they need only be alert to profit opportunities. Kirznerian entrepreneurs need not be charismatic leaders, do not innovate, and are not necessarily creative or in possession of sound business judgment. They do not necessarily start firms, raise capital, or manage an enterprise. They perform the discovery function, and nothing else. Kirzner's work is routinely invoked in references to the classics of the economics of entrepreneurship, alongside Knight's (1921) and Schumpeter's (1911) contributions. Kirzner's framework builds on the market-process approach associated with the Austrian school of economics and can trace its roots further back to Richard Cantillon, J. B. Clark, Frank A. Fetter, and other writers. Kirzner himself sees his contribution as primarily an extension of the work of Mises and F. A. Hayek, in effect bridging Mises's (1949) emphasis on the entrepreneur with Hayek's (1946, 1968) conceptualization of market competition as an unfolding process of discovery and learning. Among mainstream economists, Kirzner has been cited in the literature on occupational choice, and there have been a few attempts to formalize his model of the market process (Littlechild, 1979; Yates, 2000) and a few experimental investigations (Demmert and Klein, 2003; Kitzmann and Schiereck, 2005). Kirzner has explained the Austrian model of the entrepreneurial market process to readers of the prestigious Journal of Economic Literature (Kirzner, 1997). Still, his work has been more influential among management scholars than among economists. Thus, his approach underlies much of the opportunity-discovery or opportunity-recognition branch of the modern entrepreneurship literature (Shane and Venkataraman, 2000; Gaglio and Katz, 2001; Shane, 2003) which makes opportunities, and their discovery and (potential) exploitation, the unit of analysis for entrepreneurship research. This has given rise to a large theoretical, empirical, and experimental literature looking into the various antecedents of such opportunity discovery. As we shall argue, this literature goes much beyond Kirzner's work, making opportunity discovery and its determinants the key feature of the theory, whereas Kirzner's real interest lies in explaining market equilibration, a higher-level phenomenon. This chapter traces the origin and development of the concept of entrepreneurial alertness. In particular, we place Kirzner's contribution within the broader context of the Austrian school of economics, comparing and contrasting it with other Austrian conceptions of entrepreneurship. We argue that while Kirzner's contribution is often thought of as the Austrian conception of the entrepreneur, there is in fact an alternative Austrian tradition that emphasizes the entrepreneur as an uncertainty-bearing, asset-owning individual and that this tradition offers some advantages over the discovery approach (whether in the Kirznerian or modern-management incarnations). We also critically discuss the way Kirzner's work has been interpreted and used in the opportunity discovery approach in recent management research on entrepreneurship.