This dissertation deals with the German interest barrier ( 4h EStG) which was inserted in the German Income Tax Act by the Business Tax Amendment Act 2008 (Unternehmensteuerreformgesetz 2008). The interest barrier limits the deductibility of interest expenditures to 30 % of the clearable EBITDA. Thus the legislator intends to restrict profit shifting abroad and to counter an erosion of the German tax base. Companies which are not part of group are excempted. The relevance of the definition of "group" is not only limited to the German Income Tax Act but also has an important effect on the German Corporate Income Tax and the German Inheritance Tax Axt. The author focusses on the confusing structure of the interest barrier and gives practical guidance.