This book addresses the causes of tax policy changes, its direction and magnitude, in a comparative way by studying three dimensions: centralization, progressivity, and level in Argentina and Brazil during the 1980s and 1990s. I present a contingent application of veto players theory to explain changes in taxation. It gives importance to strategic actors' interactions and their institutional realities. I put forth the necessity of a reduction of effective veto players for change to take place within a realm of multiple veto players. I argue that due to a reduction of veto players during the late 1980s in both cases, change took place in Argentina and Brazil, but to different extent and nature. The reason for their divergences is the different nature of the respective veto players' reductions and the prevailing interests after these temporary reductions. The result of the analysis is that Argentina and Brazil decreased the progressive capacity of their taxation, but Argentina decreased it more than Brazil. I close this book with an empirical and theoretical puzzle regarding Argentina's lower taxation level than Brazil.