The Japanese population is aging faster than any other in the world. The per­ centage of Japans population aged 65 and above was only 7.1% in 1970,but just 30 years later, in 2000, it reached 17.2%. A declining birth rate and a rising average life expectancy will continue to push this trend further. This situation is causing serious problems for Japanese society.Structural reforms, especially tax and social security reforms, to accommodate this drastic demographic change have become an urgent policy issue. The purpose of this book is to establish guidelines for tax and social security reforms in Japan in terms that are both efficient and equitable. In this study, an extended life-cycle general equilibrium model is employed to rigorously take account of the rapidly aging Japanese population. The simulation approach adopted in our analysis permits us to calculate the effects of alternative policy packages on capital accumulation and economic welfare. This enables us to make proposals for concrete economic policies.