In the election to the House of Representatives in 2009, the Democratic Party of Japan (DPJ) almost tripled the number of its lower house members by winning 308 seats. It subsequently formed a coalition government with the Social Democratic Party and the People’s New Party. The new ruling party promised to completely overhaul policymaking mechanisms that had been shaped over the past decades. Yet, the Japanese people quickly felt disappointed with the DPJ’s ‘policymaking engineering’. Examining the evolution of the decision-making process in Japan under the DPJ administration between the years 2009-2012, this book offers a multidimensional explanation for the reasons for the DPJ’s failure in producing effective policymaking mechanisms. Implementing conceptual tools borrowed from historical institutionalism, the author explains why the Democrats displayed inflexibility in introducing selected elements of the Westminster system, incoherence in regard to many aspects of the decision-making reform, and unwillingness to take advantage of all of the institutional resources at their disposal. The book argues that the examination of the DPJ’s origins and interactions with other parties is crucial in understanding its misconceptions regarding the institutional model, policy vision, and institutional tools required for a durable change in policymaking patterns. Illustrating its argument with a range of case studies, this book explains why, ultimately, the DPJ’s concept of a politician-led government resulted in failure. It will also be helpful in understanding the prerequisites for the success of institutional reforms in general. As such it will be of interest to students and scholars of Japanese studies, Political science, Asian studies.