The study of policy instruments dates from the early seventies, though there has been written a lot before especially in economics about government intervention in relation to market imperfections. A policy instrument refers the means of government intervention in markets or, in broader perspective, society in order to accomplish goals or to solve problems. The behavioral assumption underlying a policy instrument is that it attempts to get people do things that they might not otherwise have done. In the last fifty years we featured a transformation not just in the scope and scale of the role of the government, but also in the proliferation of tools that it has to its disposal for public action. In retrospect a distinction can made between three partly overlapping stadiums in the study of policy instruments.