Leonid Hurwicz (1917-2008) was a major figure in modern theoretical economics whose contributions over sixty-five years spanned at least five areas: econometrics, nonlinear programming, decision theory, microeconomic theory, and mechanism design. In 2007, at age ninety, he received the Nobel Memorial Prize in Economics (shared with Eric Maskin and Roger Myerson) for pioneering the field of mechanism design and incentive compatibility. Hurwicz made seminal contributions in the other areas as well. In non-linear programming, he contributed to the understanding of Lagrange-Kuhn-Tucker problems (along with co-authors Kenneth Arrowand Hirofumi Uzawa). In econometrics, the Hurwicz bias in the least-squares analysis of time series is a fundamental and commonly cited benchmark. In decision theory, the Hurwicz criterion for decision-making under ambiguity is routinely invoked, sometimes without a citation since his original paper was never published. In microeconomic theory, Hurwicz (along with Arrow and H.D. Block) initiated the study of stability of the market mechanism, and (with Uzawa) solved the classic integrability of demand problem, a core result in neoclassical consumer theory. While some of Hurwicz's work were published in journals, many remain scattered as chapters in books which are difficult to access; yet others were never published at all. The Collected Papers of Leonid Hurwicz is the first volume in a series of four that will bring his oeuvre in one place, to bring to light the totality of his intellectual output, to document his contribution to economics and the extent of his legacy, with the express purpose to make it easily available for future generations of researchers to build upon.