The increased mobility of economic activities has resulted in a sharp increase in tax competition between countries. Tax competition can have desirable consequences, such as more efficiency, but it can also have undesirable or harmful consequences, such as a race to the bottom. Since 1997 - the year of publication of the Code of Conduct -the fight against harmful tax competition is one of the main issues of the EU-tax policy. However, the discussion about harmful tax competition till now has been very misty. For a long time the discussion about harmful tax competition had a very political character. However, in course of time the relation between harmful tax competition and the state-aids provisions in the EC-Treaty was accentuated more and more. However, a more fundamental comparison between harmful tax competition and the state-aids provisions was missing. Furthermore, the relationship between the possible measures to combat harmful tax competition and the EC Treaty did not get a lot of attention. The aim of this publication is to bring more clearness and line in the ample discussion.