公共政策是藉由重大公共議題的討論來分析政府活動的一種研究途徑，將公共政策的過程切割成議題設定、政策規劃、政策合法化、政策執行、政策評估及政策終結等階段，雖可以獲致清晰的流程架構，但此一單向的線性思考方式，不但簡化了政策問題解決的複雜性，亦會喪失政策過程的動態本質。再者，公共政策的研究領域日益蓬勃壯大，研究途徑與方法亦漸趨多元發展，理論概念與分析架構的整合自然日趨重要，如此將有助於研究者將研究議題與經驗資料作最佳的組合。是以，本研究採取非線性的政策動態概念，對政策的演變作一有系統性的探索，在融合有關政策變遷的各種觀點後，提出「政策體系」的理論概念，建立「宏觀」的分析架構作為探索政策變遷現象的基礎。 政策體系乃是以政策過程為焦點，所有與政策性質相關的要素，都可視為政策體系的構成要素。首先，政策是在開放的體系中形成、維持、接續與終結，自環境中汲取養分同時受到環境的限制，由此觀之，外在環境因素是政策體系中不可忽略的一環。其次，討論政策體系自然不能遺漏政策本身的因素，政策乃為解決社會問題而生，沒有社會問題的存在，當然就不會有政策制定的需求，另一方面，政策本身的組構方式則是攸關問題的解決與否。再者，在民主體制中，任何的政策形成與改變都必須取得正當性的依據，只有問題的存在並不保證一定會被政府所處理，必須通過政治結構的考驗才可能有政策產出。最後，政策體系是一個互動的體系，政策參與者之間就政策方案的選擇所從事的各種行為與活動，都可能影響最後的政策產出。歸納言之，一個完整的政策體系至少應該包括幾個要素：外在環境的因素、政策本身的特性、結構性因素的作用及政策行動者的互動行為，這些要素的互動作用會導致政策的變遷。 最後，政策是政府選擇目標與工具來解決社會問題的重要活動，它不是在真空中運作，而是不斷與社會進行互動的連續性過程，這也是政策走進歷史之前會出現變遷的原因，本研究雖然提出具有規範意涵的分析政策變遷的理論架構，但是基於「真實個案是檢驗理論效度的最佳途徑」的法則，本研究以民進黨首次執政時期所推動的停建核四政策個案為例，利用文本中所建立的分析架構來詮釋此一政策變遷的過程，並在結論中整合理論與個案的研究發現，就政策體系的各個面向提出幾個有關政策變遷的假設性命題，以供後續研究之參考與檢證，進而促成政策變遷知識的深化。 / Public policy is a research approach to analyze governmental activities by discussing public issues. However, in most articles, the policy process is seen as a logical succession of steps: agenda setting, policy formulation, policy legitimation, policy implementation, policy evaluation, and policy termination. Although this progression of stages provides a clear and useful framework, it often obscures the complexities of public issues. In other words, most policies are not a process of linear development, from agenda setting to policy termination. Policy change is a common phenomenon. In this way, if we want to understand the dynamics of policy, it is helpful to take a nonlinear perspective. Policy change is used by a wide range of authors in public policy and other disciplines, but there is no consensus on what the definition of policy change is. In fact, this question has not been systematically explored until now. Policy change is not simply a label, but is an important part of the policy process. It is worth drawing more attention to this phenomenon. The aim of this study is to construct an analytical framework of policy change. Within this framework, we distinguish between different types of policy change: policy succession and policy termination, firstly. And we can find some cues from policy change, including organization, statutes, budget, personnel, and media. Secondly, we use the concept of policy system to connect with policy change. This concept contains several elements: external environment, policy attributes, institutional factor, and policy networks. These elements interact with each other to produce policy change. We use this framework to analyze policy discontinuity for the fourth nuclear power plant in 2000. Finally, policy is essentially about government. The major activity of government is seen as choosing goals and instruments to solve social problems. Policy does not exist in a vacuum; it is made in a concrete environment. We should see policy as a continuing process of social action and interaction. This is why policy may be changed before it is terminated. Finally, we provide some hypotheses and suggestions for further research at the end of this study.