State aid is any aid granted by a Member State or through State resources in any form whatsoever which distorts or threatens to distort competition by favouring certain undertakings or the production of certain goods shall, in so far as it affects trade between Member States. State aid rules are an important part of EU law because they prevent subsidy race between Member States and protect an internal market. It is considered that a measure favours certain undertakings by providing them with an advantage if it leads to an improvement of the economic and/or financial position of the beneficiary and if the beneficiary could not obtain that kind of an improvement under normal market conditions. In order to determine whether a certain measure improved the economic and/or financial position of the beneficiary it is necessary to compare its situation to that of other undertakings on the market. In order to determine whether that kind of an improvement would be possible under normal market conditions we have to compare the behaviour of State as potential provider of an aid in certain case with the behaviour of a private investor under similar circumstances: if a private investor under similar circumstances would have made the same decision as the one that is suspected to be State aid, there is no advantage and consequently there is no State aid. In case of repayment agreements concluded between public authorities and undertaking State's behaviour has to be compared with the one of private creditor: if the undertaking would not have obtained comparable financing facilities from a private creditor in the same situation, there is an advantage. In last several years there were many cases that developed the notion of an advantage. These cases show that the Court is more in favour of unambiguous and stricter rules when determining whether a certain measure represents State aid. On the other hand, the Commission takes a more economical standing.