The thesis tries to investigate, how rational behaving tax auditors influence a theoretical model of tax evasion. Firstly a brief terminology is provided followed by a general introduction on the area of research. This is followed by some theoretical models of tax evasion such as the base model of Allingham and Sandmo, two different strategic models with a one-period horizon as well as an outline of a dynamic approach which has an infinite time horizon. Furthermore a model for companies which are controlled by a professional management, is introduced and the implications of an independent tax agency are analysed. The introduction closes with a model which investigates tax evasion in combination with corrupt tax officers. The main part of the thesis concentrates on integrating a rational behaving tax auditor into a binary strategic model of tax evasion, which originally only contains a taxpaying individual and the tax authority. In a first step the tax auditor is able to choose whether to use a perfect technology of auditing or not. With the further development of the made extension the tax auditor can even choose the result, which he would like to report to the other players. As the analysis of the model is done in several steps, in the first step a situation is investigated, where the tax auditor is the single and final person who can determine the payment of the taxpayer. In a second step the tax authority is provided with a mechanism in order to revise the work of the tax auditor. After that a situation is analysed, where only the taxpayer can file a protest against the judgement of the tax auditor at an independent court. Finally the thesis presents a model, where both other players are able to use a mechanism in order to examine the work of the tax auditor.