In this paper, the first part of our three-part study on the development of brownfields policy in Wisconsin, we examine the regulatory history of the brownfields policy. We start with the 1978 Hazardous Substance Spill Law, the antecedent to the brownfields regulatory reform of the 1990s, and examine the interaction of policy entrepreneurs in both the public and the private sectors that has led to innovation. We follow this by exploring the response of the Wisconsin Department of Natural Resources to reform efforts, looking at both how it anticipated and led some of the efforts and how it addressed demands placed on it by the state legislature and executive. We then discuss the central role that the state's Brownfields Study Group has played in moving brownfields cleanup and redevelopment objectives into legislation and the field. We base our work on interviews with nearly 70 individuals from public, private for-profit, private nonprofit, and tribal organizations.