New Environmental Policy Instruments (NEPIs) are becoming increasingly attractive. From a global perspective, there has been a rapid diffusion of these market-based, voluntary or informational instruments. This article examines the spread of four different NEPIs -eco-labels, energy or carbon taxes, national environmental policy plans or strategies for sustainable development, and free-access-of-information (FAI) provisions. The adoption of NEPIs by national policy makers is not simply a reaction to newly emerging environmental problems or to real or perceived deficits of traditional command and control regulation, rather the use of NEPIs can also be ascribed to the inner dynamics of international processes of policy transfer or policy diffusion. These processes make it increasingly difficult for national policy makers to ignore new approaches in environmental policy that have already been put into practice in 'forerunner' countries.

During the 1990s, a new regulatory pattern in domestic environmental policy making emerged. This pattern is largely a result of policy diffusion. In the absence of formal obligations, regulatory instruments that have been communicated internationally and were already being practiced elsewhere were voluntarily emulated and adopted by policy makers. While the international promotion of regulatory instruments often facilitated their diffusion, the instruments’ characteristics determined the extent and speed by which regulatory instruments spread across countries. The voluntary adoption of regulatory instruments cannot be exclusively explained by the rational motivation of policy makers to improve effectiveness. In addition, they were motivated by concerns of legitimacy and perceived pressure to conform with international norms.

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Comparative policy analysis increasingly faces the challenge of incorporating external forces on national policy developments into its analytical framework. Scholars of international relations have recognized that the behaviour of states -in terms of policy outputs -converges even in the absence of binding international agreements. These two branches of research can be bridged by the concept of policy diffusion. Diffusion analysis asks for those conditions that favour or hinder the spread of policy innovations within the international system. However, the scientific community struggles with different meanings and notions of the term policy diffusion. Thus, the aim of this paper is to offer a general conceptual framework for the study of diffusion processes within the international system. Understanding the process of policy diffusion requires an analysis of the complex interplay between transnational and international forces, national factors and the characteristics of policy innovations. By providing a conceptual framework this paper hopes not only to contribute to the theoretical debate but also to give guidance for empirical research.

Although the goals of the country's energy transition (Energiewende) are widely accepted in Germany, the specific route to get there is itself a matter of great controversy. The individual measures that are part of the energy transition policy and the questions of how they interact and how they are embedded in the European context are objects of controversial scientific and public debate. Most recently, the consequences for the price of electricity have, in particular, been discussed intensely. Against this backdrop of wide-ranging criticism, the future course for promoting renewable energy will soon be set. The German Renewable Energy Sources Act (the Erneuerbare-Energien-Gesetz, EEG), which is the main instrument of energy transition policy with its feed-in tariffs, is supposed to be fundamentally revised in the course of this year. A precondition for achieving a coherent further development of the energy transition policy and for receiving the sound support of a critical public is that the long-term consequences of political decisions on a complex sociotechnical energy system be taken into account. The requirements of such a system are not satisfied by policy approaches or recommendations that target short-term effects or that are perceptions of problems extrapolated from individual sectors. On the basis of its integrated research on the energy transition, researchers from the Helmholtz Alliance Energy-Trans take a stand on current important controversial issues from the energy transformation and specify fundamental challenges to shaping a sustainable energy transition policy.Keywords: Energy transition; Sustainability; Germany; European Union; State aid; Renewable energy sources; Feed-in tariff; Federalism; Energy policy; Market designThe energy transition: a long-term project and a challenge to the system The energy transition in Germany is nothing less than the restructuring of the entire energy supply in the sectors' electricity, heat, and transportation in a highly industrialized country. This comprehensive reorganization is a task for generations, and yet the fundamental framework has to be created today. Although there continues to be a high level of approval in the general public [1][2][3][4] and among all political forces [5], essential components of the German energy transition policy are themselves currently being subject to criticism, some of it very intense. The focus of this criticism is most frequently on the renewable energy supports provided for the generation of electricity, which take the form of feed-in tariffs permitted by the Renewable Energy Sources Act (Erneuerbare-Energien- Gesetz (EEG)).a Challenges to the energy transition go, however, far beyond this. What is required is a long-term process of transforming a complex sociotechnical system [6,7] in which the goal is to set the course of change so that tomorrow's energy supply works, its consumption of resources, and its impact on the environment are limited to a sustainable level, and, in the process, efficiency and social ac...