Taxes are major source of public funds to finance government expenditures. Tax authorities impose different kind of taxes and employ many agents to collect taxes effectively. Some dutiful taxpayers will undoubtedly pay their tax liabilities while many others will not. The Internal Revenue Service in the United States reports that the estimate of income tax liability not collected is about 17, which translates into 345 billion for 2001. It is important to make a distinction between tax evasion and tax avoidance. The distinguishing characteristic of evasion is illegality. Whether the reason for not paying tax liability is avoidance or evasion, economic models of taxation need to be changed in the light of these realities. In this study, I analyze some of the economic problems of tax evasion/avoidance. In the first chapter, I discuss the relationship between number of tax audits, tax administration reform and tax compliance in Turkey. In recent years, many developing countries have carried out reforms in their tax administration to increase their efficiency in collecting taxes. In 2005, the tax authority in Turkey established Tax Office Directorates (T.O.D.s) in 29 provinces for the purpose of controlling the underground economy, improving taxpayer assistance, and increasing auditing efficiency. By using the panel data on province level tax returns, my analysis answers two questions. First, I examine the effect of audits on reported income and reported tax liability. By controlling for the detectibility of evasion and other socioeconomic variables, I find that audits have the same effectiveness in increasing reported income and reported tax liability. Second, I investigate the effect of establishing T.O.D.s in 29 provinces on compliance in those provinces. I find that T.O.D.s are effective at the extensive margin rather than the intensive margin. Thus, establishing T.O.D.s had no significant effect on the compliance level of existing taxpayers while it increased the number of tax returns significantly. In the second chapter, I analyze the excess burden on income tax when tax avoidance matters. I present a simple static labor supply model with endogenous asset choice. Then, I examine how tax avoidance through asset trading a ects the labor supply response and the excess burden of income tax. Furthermore, I discuss the implications of the tax policy analysis and show that a failure to account for avoidance responses may lead to errors when estimating how tax reform affects labor supply, tax revenue, and the welfare cost of taxation. Because of tax avoidance through tax arbitrage, the progressivity of a given tax system will be less than what the formal tax system implies. In the third chapter,we study the Marginal Cost of Funds in the existence of tax evasion. We develop a general equilibrium model of tax evasion, including the expected utility of taxpayers and three different revenue-raising government policies. In this rich model environment, we analytically derive the marginal cost of funds (MCF) for the alternative policy instruments. We consider two main fiscal reforms: the revision in the nonlinear tax scheme and the changes in enforcement mechanism (the audit and penalty rates). First, we derive the MCF for the tax reform and find its key determinants. The derived MCF is greater than the previous ones since it includes a "risk-bearing cost" as well as tax distortion. The reform in enforcement mechanism generates MCFs in different forms. Two more MCFs with respect to audit and penalty rates are presented. Finally, we compare these three different MCFs in numerical example and provide some policy implications. / text