Fiscal policy plays a key role in the economic development prospects. Fiscal policy is an important instrument of macroeconomic policy, and consists of government spending and tax policies. Governments use fiscal policy to influence the level of aggregate demand in the economy, in an effort to achieve economic objectives of price stability, full employment and economic growth. The essential argument is that the economy's ability to generate revenues remains very limited. The existence of tax systems has emerged as a necessity of state to finance the public goods and services that it offers to its citizens. The proceeds of the tax system constitute the main potential revenues for most states. This book aims to give a clear picture of the concepts and terms associated with the tax system. In this book are presented mainly thoughts and studies by economists of flat and progressive tax, are also presented evidence concerning the application of a model of taxation in different countries, different debates and various researches have related with one or another tax.